Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020-12-22 11:16, Masahiro Ikeda wrote:
Thanks for your comments.
On 2020-12-22 09:39, Andres Freund wrote:
Hi,
On 2020-12-21 13:16:50 -0800, Andres Freund wrote:
On 2020-12-02 13:52:43 +0900, Fujii Masao wrote:
> Pushed. Thanks!
Why are wal_records/fpi long, instead of uint64?
longwal_records;/* # of WAL records produced */
longwal_fpi;/* # of WAL full page images
produced */
uint64 wal_bytes; /* size of WAL records produced
*/
long is only 4 byte e.g. on windows, and it is entirely possible to
wrap
a 4 byte record counter. It's also somewhat weird that wal_bytes is
unsigned, but the others are signed?
This is made doubly weird because on the SQL level you chose to make
wal_records, wal_fpi bigint. And wal_bytes numeric?
I'm sorry I don't know the reason.
The following thread is related to the patch and the type of wal_bytes
is changed from long to uint64 because XLogRecPtr is uint64.
https://www.postgresql.org/message-id/flat/20200402144438.GF64485%40nol#1f0127c98df430104c63426fdc285c20
I assumed that the reason why the type of wal_records/fpi is long
is BufferUsage have the members (i.e, shared_blks_hit) of the same
types.
So, I think it's better if to change the type of wal_records/fpi from
long to uint64,
to change the types of BufferUsage's members too.
I've done a little more research so I'll share the results.
IUCC, theoretically this leads to caliculate the statistics less,
but actually, it's not happened.
The above "wal_records", "wal_fpi" are accumulation values and when
WalUsageAccumDiff()
is called, we can know how many wals are generated for specific
executions,
for example, planning/executing a query, processing a utility command,
and vacuuming one relation.
The following variable has accumulated "wal_records" and "wal_fpi" per
process.
```
typedef struct WalUsage
{
longwal_records;/* # of WAL records produced */
longwal_fpi;/* # of WAL full page images
produced */
uint64 wal_bytes; /* size of WAL records produced
*/
} WalUsage;
WalUsagepgWalUsage;
```
Although this may be overflow, it doesn't affect to caliculate the
difference
of wal usage between some execution points. If to generate over 2
billion wal
records per executions, 4 bytes is not enough and collected statistics
will be
lost, but I think it's not happened.
In addition, "wal_records" and "wal_fpi" values sent by processes are
accumulated in the statistic collector and their types are
PgStat_Counter(int64).
```
typedef struct PgStat_WalStats
{
PgStat_Counter wal_records;
PgStat_Counter wal_fpi;
uint64 wal_bytes;
PgStat_Counter wal_buffers_full;
TimestampTz stat_reset_timestamp;
} PgStat_WalStats;
```
Some more things:
- There's both PgStat_MsgWal WalStats; and static PgStat_WalStats
walStats;
that seems *WAY* too confusing. And the former imo shouldn't be
global.
Sorry for the confusing name.
I referenced the following variable name.
static PgStat_MsgSLRU SLRUStats[SLRU_NUM_ELEMENTS];
static PgStat_SLRUStats slruStats[SLRU_NUM_ELEMENTS];
How about to change from "PgStat_MsgWal WalStats"
to "PgStat_MsgWal MsgWalStats"?
Is it better to change the following name too?
"PgStat_MsgBgWriter BgWriterStats;"
"static PgStat_MsgSLRU SLRUStats[SLRU_NUM_ELEMENTS];"
Since PgStat_MsgWal is called in xlog.c and pgstat.c,
I thought it's should be global.
I made an attached patch to rename the above variable names.
What do you think?
Regards,
--
Masahiro Ikeda
NTT DATA CORPORATIONFrom 8bde948e5e91dbfbcf79b091af51f022aa32191a Mon Sep 17 00:00:00 2001
From: Masahiro Ikeda
Date: Wed, 20 Jan 2021 12:13:24 +0900
Subject: [PATCH] Refactor variable names of global statistics messages
Refactor the variable names of global statistics messages
for bgwriter, wal, and SLRU because the names are too confusing.
Now, their names are BgWriterStats, WalStats, and SLRUStats. But,
there are alike names that are defined in the statistic collector.
Their names are walStats and slruStats. Since this is confusing,
this patch renames variable names of messages to make it easy to
understand that they are messages.
Author: Masahiro Ikeda
Reviewed-by: Andres Freund
Discussion: https://postgr.es/m/20201222003935.47aoxfmokltlr...@alap3.anarazel.de
---
src/backend/access/transam/xlog.c | 6 ++--
src/backend/postmaster/checkpointer.c | 8 +++---
src/backend/postmaster/pgstat.c | 40 +--
src/backend/storage/buffer/bufmgr.c | 8 +++---
src/include/pgstat.h | 4 +--
5 files changed, 33 insertions(+), 33 deletions(-)
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 470e113b33..a64ad34f21 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
Thanks for your comments. On 2020-12-22 09:39, Andres Freund wrote: Hi, On 2020-12-21 13:16:50 -0800, Andres Freund wrote: On 2020-12-02 13:52:43 +0900, Fujii Masao wrote: > Pushed. Thanks! Why are wal_records/fpi long, instead of uint64? longwal_records;/* # of WAL records produced */ longwal_fpi;/* # of WAL full page images produced */ uint64 wal_bytes; /* size of WAL records produced */ long is only 4 byte e.g. on windows, and it is entirely possible to wrap a 4 byte record counter. It's also somewhat weird that wal_bytes is unsigned, but the others are signed? This is made doubly weird because on the SQL level you chose to make wal_records, wal_fpi bigint. And wal_bytes numeric? I'm sorry I don't know the reason. The following thread is related to the patch and the type of wal_bytes is changed from long to uint64 because XLogRecPtr is uint64. https://www.postgresql.org/message-id/flat/20200402144438.GF64485%40nol#1f0127c98df430104c63426fdc285c20 I assumed that the reason why the type of wal_records/fpi is long is BufferUsage have the members (i.e, shared_blks_hit) of the same types. So, I think it's better if to change the type of wal_records/fpi from long to uint64, to change the types of BufferUsage's members too. Some more things: - There's both PgStat_MsgWal WalStats; and static PgStat_WalStats walStats; that seems *WAY* too confusing. And the former imo shouldn't be global. Sorry for the confusing name. I referenced the following variable name. static PgStat_MsgSLRU SLRUStats[SLRU_NUM_ELEMENTS]; static PgStat_SLRUStats slruStats[SLRU_NUM_ELEMENTS]; How about to change from "PgStat_MsgWal WalStats" to "PgStat_MsgWal MsgWalStats"? Is it better to change the following name too? "PgStat_MsgBgWriter BgWriterStats;" "static PgStat_MsgSLRU SLRUStats[SLRU_NUM_ELEMENTS];" Since PgStat_MsgWal is called in xlog.c and pgstat.c, I thought it's should be global. - AdvanceXLInsertBuffer() does WalStats.m_wal_buffers_full, but as far as I can tell there's nothing actually sending that? IIUC, when pgstat_send_wal() is called by backends and so on, it is sent to the statistic collector and it is exposed via pg_stat_wal view. Regards, -- Masahiro Ikeda NTT DATA CORPORATION
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
Hi, On 2020-12-21 13:16:50 -0800, Andres Freund wrote: > On 2020-12-02 13:52:43 +0900, Fujii Masao wrote: > > Pushed. Thanks! > > Why are wal_records/fpi long, instead of uint64? > longwal_records;/* # of WAL records produced */ > longwal_fpi;/* # of WAL full page images > produced */ > uint64 wal_bytes; /* size of WAL records produced > */ > > long is only 4 byte e.g. on windows, and it is entirely possible to wrap > a 4 byte record counter. It's also somewhat weird that wal_bytes is > unsigned, but the others are signed? > > This is made doubly weird because on the SQL level you chose to make > wal_records, wal_fpi bigint. And wal_bytes numeric? Some more things: - There's both PgStat_MsgWal WalStats; and static PgStat_WalStats walStats; that seems *WAY* too confusing. And the former imo shouldn't be global. - AdvanceXLInsertBuffer() does WalStats.m_wal_buffers_full, but as far as I can tell there's nothing actually sending that? - Andres
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
Hi, On 2020-12-02 13:52:43 +0900, Fujii Masao wrote: > Pushed. Thanks! Why are wal_records/fpi long, instead of uint64? longwal_records;/* # of WAL records produced */ longwal_fpi;/* # of WAL full page images produced */ uint64 wal_bytes; /* size of WAL records produced */ long is only 4 byte e.g. on windows, and it is entirely possible to wrap a 4 byte record counter. It's also somewhat weird that wal_bytes is unsigned, but the others are signed? This is made doubly weird because on the SQL level you chose to make wal_records, wal_fpi bigint. And wal_bytes numeric? Greetings, Andres Freund
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020/12/01 14:01, Fujii Masao wrote:
On 2020/11/26 16:07, Masahiro Ikeda wrote:
On 2020-11-25 20:19, Fujii Masao wrote:
On 2020/11/19 16:31, Masahiro Ikeda wrote:
On 2020-11-17 11:46, Fujii Masao wrote:
On 2020/11/16 16:35, Masahiro Ikeda wrote:
On 2020-11-12 14:58, Fujii Masao wrote:
On 2020/11/06 10:25, Masahiro Ikeda wrote:
On 2020-10-30 11:50, Fujii Masao wrote:
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both number
are important.
Also about performance, I thought there are few impacts because it
increments stats in memory. If I can implement to reuse pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
Yes, thanks. I fixed it.
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two for
walwriter and other processes. Instead, we can use one m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the counters
per process type if necessary. If we adopt this approach, we can easily
extend pg_stat_wal so that any fields can be reported per process type.
I'll remove the above source code because these counters are not useful.
On 2020-10-30 12:00, Fujii Masao wrote:
On 2020/10/20 11:31, Masahiro Ikeda wrote:
Hi,
I think we need to add some statistics to pg_stat_wal view.
Although there are some parameter related WAL,
there are few statistics for tuning them.
I think it's better to provide the following statistics.
Please let me know your comments.
```
postgres=# SELECT * from pg_stat_wal;
-[ RECORD 1 ]---+--
wal_records | 2000224
wal_fpi | 47
wal_bytes | 248216337
wal_buffers_full | 20954
wal_init_file | 8
wal_write_backend | 20960
wal_write_walwriter | 46
wal_write_time | 51
wal_sync_backend | 7
wal_sync_walwriter | 8
wal_sync_time | 0
stats_reset | 2020-10-20 11:04:51.307771+09
```
1. Basic statistics of WAL activity
- wal_records: Total number of WAL records generated
- wal_fpi: Total number of WAL full page images generated
- wal_bytes: Total amount of WAL bytes generated
To understand DB's performance, first, we will check the performance
trends for the entire database instance.
For example, if the number of wal_fpi becomes higher, users may tune
"wal_compression", "checkpoint_timeout" and so on.
Although users can check the above statistics via EXPLAIN, auto_explain,
autovacuum and pg_stat_statements now,
if users want to see the performance trends for the entire database,
they must recalculate the statistics.
I think it is useful to add the sum of the basic statistics.
2. WAL segment file creation
- wal_init_file: Total number of WAL segment files created.
To create a new WAL file may have an impact on the
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020/11/26 16:07, Masahiro Ikeda wrote:
On 2020-11-25 20:19, Fujii Masao wrote:
On 2020/11/19 16:31, Masahiro Ikeda wrote:
On 2020-11-17 11:46, Fujii Masao wrote:
On 2020/11/16 16:35, Masahiro Ikeda wrote:
On 2020-11-12 14:58, Fujii Masao wrote:
On 2020/11/06 10:25, Masahiro Ikeda wrote:
On 2020-10-30 11:50, Fujii Masao wrote:
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both number
are important.
Also about performance, I thought there are few impacts because it
increments stats in memory. If I can implement to reuse pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
Yes, thanks. I fixed it.
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two for
walwriter and other processes. Instead, we can use one m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the counters
per process type if necessary. If we adopt this approach, we can easily
extend pg_stat_wal so that any fields can be reported per process type.
I'll remove the above source code because these counters are not useful.
On 2020-10-30 12:00, Fujii Masao wrote:
On 2020/10/20 11:31, Masahiro Ikeda wrote:
Hi,
I think we need to add some statistics to pg_stat_wal view.
Although there are some parameter related WAL,
there are few statistics for tuning them.
I think it's better to provide the following statistics.
Please let me know your comments.
```
postgres=# SELECT * from pg_stat_wal;
-[ RECORD 1 ]---+--
wal_records | 2000224
wal_fpi | 47
wal_bytes | 248216337
wal_buffers_full | 20954
wal_init_file | 8
wal_write_backend | 20960
wal_write_walwriter | 46
wal_write_time | 51
wal_sync_backend | 7
wal_sync_walwriter | 8
wal_sync_time | 0
stats_reset | 2020-10-20 11:04:51.307771+09
```
1. Basic statistics of WAL activity
- wal_records: Total number of WAL records generated
- wal_fpi: Total number of WAL full page images generated
- wal_bytes: Total amount of WAL bytes generated
To understand DB's performance, first, we will check the performance
trends for the entire database instance.
For example, if the number of wal_fpi becomes higher, users may tune
"wal_compression", "checkpoint_timeout" and so on.
Although users can check the above statistics via EXPLAIN, auto_explain,
autovacuum and pg_stat_statements now,
if users want to see the performance trends for the entire database,
they must recalculate the statistics.
I think it is useful to add the sum of the basic statistics.
2. WAL segment file creation
- wal_init_file: Total number of WAL segment files created.
To create a new WAL file may have an impact on the performance of
a write-heavy workload
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020-11-25 20:19, Fujii Masao wrote:
On 2020/11/19 16:31, Masahiro Ikeda wrote:
On 2020-11-17 11:46, Fujii Masao wrote:
On 2020/11/16 16:35, Masahiro Ikeda wrote:
On 2020-11-12 14:58, Fujii Masao wrote:
On 2020/11/06 10:25, Masahiro Ikeda wrote:
On 2020-10-30 11:50, Fujii Masao wrote:
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both
number
are important.
Also about performance, I thought there are few impacts
because it
increments stats in memory. If I can implement to reuse
pgWalUsage's
value which already collects these stats, there is no impact
in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the
accumulated
value of wal_records, wal_fpi, and wal_bytes to use
pgWalUsage's
value?
I don't think that works, but it would work that
pgstat_send_wal()
takes the difference of that values between two successive
calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes -
prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi -
prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes -
prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi -
prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
Yes, thanks. I fixed it.
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two
for
walwriter and other processes. Instead, we can use one
m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the
counters
per process type if necessary. If we adopt this approach, we can
easily
extend pg_stat_wal so that any fields can be reported per process
type.
I'll remove the above source code because these counters are not
useful.
On 2020-10-30 12:00, Fujii Masao wrote:
On 2020/10/20 11:31, Masahiro Ikeda wrote:
Hi,
I think we need to add some statistics to pg_stat_wal view.
Although there are some parameter related WAL,
there are few statistics for tuning them.
I think it's better to provide the following statistics.
Please let me know your comments.
```
postgres=# SELECT * from pg_stat_wal;
-[ RECORD 1 ]---+--
wal_records | 2000224
wal_fpi | 47
wal_bytes | 248216337
wal_buffers_full | 20954
wal_init_file | 8
wal_write_backend | 20960
wal_write_walwriter | 46
wal_write_time | 51
wal_sync_backend | 7
wal_sync_walwriter | 8
wal_sync_time | 0
stats_reset | 2020-10-20 11:04:51.307771+09
```
1. Basic statistics of WAL activity
- wal_records: Total number of WAL records generated
- wal_fpi: Total number of WAL full page images generated
- wal_bytes: Total amount of WAL bytes generated
To understand DB's performance, first, we will check the
performance
trends for the entire database instance.
For example, if the number of wal_fpi becomes higher, users may
tune
"wal_compression", "checkpoint_timeout" and so on.
Although users can check the above statistics via EXPLAIN,
auto_explain,
autovacuum and pg_stat_statements now,
if users want to see the performance trends for the entire
database,
they must recalculate the statistics.
I think it is useful to add the sum of the basic statistics.
2. WAL segment file creation
- wal_init_file: Total number of WAL segment files created.
To create a new WAL file may have an impact on the performance
of
a write-heavy workload generating lots
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020/11/19 16:31, Masahiro Ikeda wrote:
On 2020-11-17 11:46, Fujii Masao wrote:
On 2020/11/16 16:35, Masahiro Ikeda wrote:
On 2020-11-12 14:58, Fujii Masao wrote:
On 2020/11/06 10:25, Masahiro Ikeda wrote:
On 2020-10-30 11:50, Fujii Masao wrote:
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both number
are important.
Also about performance, I thought there are few impacts because it
increments stats in memory. If I can implement to reuse pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
Yes, thanks. I fixed it.
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two for
walwriter and other processes. Instead, we can use one m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the counters
per process type if necessary. If we adopt this approach, we can easily
extend pg_stat_wal so that any fields can be reported per process type.
I'll remove the above source code because these counters are not useful.
On 2020-10-30 12:00, Fujii Masao wrote:
On 2020/10/20 11:31, Masahiro Ikeda wrote:
Hi,
I think we need to add some statistics to pg_stat_wal view.
Although there are some parameter related WAL,
there are few statistics for tuning them.
I think it's better to provide the following statistics.
Please let me know your comments.
```
postgres=# SELECT * from pg_stat_wal;
-[ RECORD 1 ]---+--
wal_records | 2000224
wal_fpi | 47
wal_bytes | 248216337
wal_buffers_full | 20954
wal_init_file | 8
wal_write_backend | 20960
wal_write_walwriter | 46
wal_write_time | 51
wal_sync_backend | 7
wal_sync_walwriter | 8
wal_sync_time | 0
stats_reset | 2020-10-20 11:04:51.307771+09
```
1. Basic statistics of WAL activity
- wal_records: Total number of WAL records generated
- wal_fpi: Total number of WAL full page images generated
- wal_bytes: Total amount of WAL bytes generated
To understand DB's performance, first, we will check the performance
trends for the entire database instance.
For example, if the number of wal_fpi becomes higher, users may tune
"wal_compression", "checkpoint_timeout" and so on.
Although users can check the above statistics via EXPLAIN, auto_explain,
autovacuum and pg_stat_statements now,
if users want to see the performance trends for the entire database,
they must recalculate the statistics.
I think it is useful to add the sum of the basic statistics.
2. WAL segment file creation
- wal_init_file: Total number of WAL segment files created.
To create a new WAL file may have an impact on the performance of
a write-heavy workload generating lots of WAL. If this number is reported high,
to reduce the number of this
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020-11-17 12:53, lchch1...@sina.cn wrote: Thanks for your comments. You're understanding is almost the same as mine. It describes when not only backends but also other backgrounds initialize a new wal page, wal buffer's space is already used and there is no space. 'Total number of times WAL data written to the disk because a backend yelled a wal buffer for an advanced wal page' Thanks for your suggestion. I wondered that users may confuse about how to use "wal_buffers_full" and how to tune parameters. I thought the reason which wal buffer has no space is important for users to tune the wal_buffers parameter. How about the following comments? 'Total number of times WAL data was written to the disk because WAL buffers got full when to initialize a new WAL page' Or what about the following? Total number of times WAL data was written to the disk, to claim the buffer page to insert new WAL data when the WAL buffers got filled up with unwritten WAL data. As my understand we can not say 'full' because every wal page mapped a special wal buffer slot. When a wal page need to be write, but the buffer slot was occupied by other wal page. It need to wait the wal buffer slot released. So i think we should say it 'occupied' not 'full'. Maybe: Total number of times WAL data was written to the disk, to claim the buffer page to insert new WAL data when the special WAL buffer occupied by other page. OK, I will change the above sentence since there are some sentences like "space occupied by", "disk blocks occupied", and so on in the documents. Do we need to change the column name from "wal_buffers_full" to another name like "wal_buffers_all_occupied"? Regards -- Masahiro Ikeda NTT DATA CORPORATION
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020-11-17 11:46, Fujii Masao wrote:
On 2020/11/16 16:35, Masahiro Ikeda wrote:
On 2020-11-12 14:58, Fujii Masao wrote:
On 2020/11/06 10:25, Masahiro Ikeda wrote:
On 2020-10-30 11:50, Fujii Masao wrote:
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both
number
are important.
Also about performance, I thought there are few impacts because
it
increments stats in memory. If I can implement to reuse
pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that
pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes -
prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi -
prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes -
prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi -
prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
Yes, thanks. I fixed it.
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two
for
walwriter and other processes. Instead, we can use one
m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the
counters
per process type if necessary. If we adopt this approach, we can
easily
extend pg_stat_wal so that any fields can be reported per process
type.
I'll remove the above source code because these counters are not
useful.
On 2020-10-30 12:00, Fujii Masao wrote:
On 2020/10/20 11:31, Masahiro Ikeda wrote:
Hi,
I think we need to add some statistics to pg_stat_wal view.
Although there are some parameter related WAL,
there are few statistics for tuning them.
I think it's better to provide the following statistics.
Please let me know your comments.
```
postgres=# SELECT * from pg_stat_wal;
-[ RECORD 1 ]---+--
wal_records | 2000224
wal_fpi | 47
wal_bytes | 248216337
wal_buffers_full | 20954
wal_init_file | 8
wal_write_backend | 20960
wal_write_walwriter | 46
wal_write_time | 51
wal_sync_backend | 7
wal_sync_walwriter | 8
wal_sync_time | 0
stats_reset | 2020-10-20 11:04:51.307771+09
```
1. Basic statistics of WAL activity
- wal_records: Total number of WAL records generated
- wal_fpi: Total number of WAL full page images generated
- wal_bytes: Total amount of WAL bytes generated
To understand DB's performance, first, we will check the
performance
trends for the entire database instance.
For example, if the number of wal_fpi becomes higher, users may
tune
"wal_compression", "checkpoint_timeout" and so on.
Although users can check the above statistics via EXPLAIN,
auto_explain,
autovacuum and pg_stat_statements now,
if users want to see the performance trends for the entire
database,
they must recalculate the statistics.
I think it is useful to add the sum of the basic statistics.
2. WAL segment file creation
- wal_init_file: Total number of WAL segment files created.
To create a new WAL file may have an impact on the performance of
a write-heavy workload generating lots of WAL. If this number is
reported high,
to reduce the number of this initialization, we
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
>> Thanks for your comments. >> >> You're understanding is almost the same as mine. >> It describes when not only backends but also other backgrounds initialize a >> new wal page, >> wal buffer's space is already used and there is no space. >> >>> 'Total number of times WAL data written to the disk because a backend >>> yelled a wal buffer for an advanced wal page' >> >> Thanks for your suggestion. >> I wondered that users may confuse about how to use "wal_buffers_full" and >> how to tune parameters. >> >> I thought the reason which wal buffer has no space is >> important for users to tune the wal_buffers parameter. >> >> How about the following comments? >> >> 'Total number of times WAL data was written to the disk because WAL buffers >> got full >> when to initialize a new WAL page' >Or what about the following? >Total number of times WAL data was written to the disk, to claim the buffer >page to insert new >WAL data when the WAL buffers got filled up with unwritten WAL data. As my understand we can not say 'full' because every wal page mapped a special wal buffer slot. When a wal page need to be write, but the buffer slot was occupied by other wal page. It need to wait the wal buffer slot released. So i think we should say it 'occupied' not 'full'. Maybe: Total number of times WAL data was written to the disk, to claim the buffer page to insert new WAL data when the special WAL buffer occupied by other page. Regards, Highgo Software (Canada/China/Pakistan) URL : www.highgo.ca EMAIL: mailto:movead(dot)li(at)highgo(dot)ca
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020/11/16 18:24, Masahiro Ikeda wrote:
On 2020-11-13 12:32, lchch1...@sina.cn wrote:
Now, pg_stat_wal supports reset all informantion in WalStats
using pg_stat_reset_shared('wal') function.
Isn't it enough?
Yes it ok, sorry I miss this infomation.
OK.
3. I do not think it's a correct describe in document for
'wal_buffers_full'.
Where should I rewrite the description? If my understanding is not
correct, please let me know.
Sorry I have not described it clearly, because I can not understand
the meaning of this
column after I read the describe in document.
And now I read the source code of walwrite and found the place where
'wal_buffers_full'
added is for a backend to wait a wal buffer which is occupied by other
wal page, so the
backend flush the old page in the wal buffer(after wait it can).
So i think the origin decribe in document is not so in point, we can
describe it such as
'Total number of times WAL data written to the disk because a backend
yelled a wal buffer
for an advanced wal page.
Sorry if my understand is wrong.
Thanks for your comments.
You're understanding is almost the same as mine.
It describes when not only backends but also other backgrounds initialize a new
wal page,
wal buffer's space is already used and there is no space.
'Total number of times WAL data written to the disk because a backend
yelled a wal buffer for an advanced wal page'
Thanks for your suggestion.
I wondered that users may confuse about how to use "wal_buffers_full" and how
to tune parameters.
I thought the reason which wal buffer has no space is
important for users to tune the wal_buffers parameter.
How about the following comments?
'Total number of times WAL data was written to the disk because WAL buffers got
full
when to initialize a new WAL page'
Or what about the following?
Total number of times WAL data was written to the disk, to claim the buffer
page to insert new WAL data when the WAL buffers got filled up with unwritten
WAL data.
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020/11/16 16:35, Masahiro Ikeda wrote:
On 2020-11-12 14:58, Fujii Masao wrote:
On 2020/11/06 10:25, Masahiro Ikeda wrote:
On 2020-10-30 11:50, Fujii Masao wrote:
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both number
are important.
Also about performance, I thought there are few impacts because it
increments stats in memory. If I can implement to reuse pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
Yes, thanks. I fixed it.
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two for
walwriter and other processes. Instead, we can use one m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the counters
per process type if necessary. If we adopt this approach, we can easily
extend pg_stat_wal so that any fields can be reported per process type.
I'll remove the above source code because these counters are not useful.
On 2020-10-30 12:00, Fujii Masao wrote:
On 2020/10/20 11:31, Masahiro Ikeda wrote:
Hi,
I think we need to add some statistics to pg_stat_wal view.
Although there are some parameter related WAL,
there are few statistics for tuning them.
I think it's better to provide the following statistics.
Please let me know your comments.
```
postgres=# SELECT * from pg_stat_wal;
-[ RECORD 1 ]---+--
wal_records | 2000224
wal_fpi | 47
wal_bytes | 248216337
wal_buffers_full | 20954
wal_init_file | 8
wal_write_backend | 20960
wal_write_walwriter | 46
wal_write_time | 51
wal_sync_backend | 7
wal_sync_walwriter | 8
wal_sync_time | 0
stats_reset | 2020-10-20 11:04:51.307771+09
```
1. Basic statistics of WAL activity
- wal_records: Total number of WAL records generated
- wal_fpi: Total number of WAL full page images generated
- wal_bytes: Total amount of WAL bytes generated
To understand DB's performance, first, we will check the performance
trends for the entire database instance.
For example, if the number of wal_fpi becomes higher, users may tune
"wal_compression", "checkpoint_timeout" and so on.
Although users can check the above statistics via EXPLAIN, auto_explain,
autovacuum and pg_stat_statements now,
if users want to see the performance trends for the entire database,
they must recalculate the statistics.
I think it is useful to add the sum of the basic statistics.
2. WAL segment file creation
- wal_init_file: Total number of WAL segment files created.
To create a new WAL file may have an impact on the performance of
a write-heavy workload generating lots of WAL. If this number is reported high,
to reduce the number of this initialization, we can tune WAL-related parameters
so that more "recycled" WAL files can
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020-11-13 12:32, lchch1...@sina.cn wrote:
Now, pg_stat_wal supports reset all informantion in WalStats
using pg_stat_reset_shared('wal') function.
Isn't it enough?
Yes it ok, sorry I miss this infomation.
OK.
3. I do not think it's a correct describe in document for
'wal_buffers_full'.
Where should I rewrite the description? If my understanding is not
correct, please let me know.
Sorry I have not described it clearly, because I can not understand
the meaning of this
column after I read the describe in document.
And now I read the source code of walwrite and found the place where
'wal_buffers_full'
added is for a backend to wait a wal buffer which is occupied by other
wal page, so the
backend flush the old page in the wal buffer(after wait it can).
So i think the origin decribe in document is not so in point, we can
describe it such as
'Total number of times WAL data written to the disk because a backend
yelled a wal buffer
for an advanced wal page.
Sorry if my understand is wrong.
Thanks for your comments.
You're understanding is almost the same as mine.
It describes when not only backends but also other backgrounds
initialize a new wal page,
wal buffer's space is already used and there is no space.
'Total number of times WAL data written to the disk because a backend
yelled a wal buffer for an advanced wal page'
Thanks for your suggestion.
I wondered that users may confuse about how to use "wal_buffers_full"
and how to tune parameters.
I thought the reason which wal buffer has no space is
important for users to tune the wal_buffers parameter.
How about the following comments?
'Total number of times WAL data was written to the disk because WAL
buffers got full
when to initialize a new WAL page'
Regards,
--
Masahiro Ikeda
NTT DATA CORPORATION
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020-11-12 14:58, Fujii Masao wrote:
On 2020/11/06 10:25, Masahiro Ikeda wrote:
On 2020-10-30 11:50, Fujii Masao wrote:
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both
number
are important.
Also about performance, I thought there are few impacts because it
increments stats in memory. If I can implement to reuse
pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes -
prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi -
prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes -
prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
Yes, thanks. I fixed it.
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two for
walwriter and other processes. Instead, we can use one
m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the
counters
per process type if necessary. If we adopt this approach, we can
easily
extend pg_stat_wal so that any fields can be reported per process
type.
I'll remove the above source code because these counters are not
useful.
On 2020-10-30 12:00, Fujii Masao wrote:
On 2020/10/20 11:31, Masahiro Ikeda wrote:
Hi,
I think we need to add some statistics to pg_stat_wal view.
Although there are some parameter related WAL,
there are few statistics for tuning them.
I think it's better to provide the following statistics.
Please let me know your comments.
```
postgres=# SELECT * from pg_stat_wal;
-[ RECORD 1 ]---+--
wal_records | 2000224
wal_fpi | 47
wal_bytes | 248216337
wal_buffers_full | 20954
wal_init_file | 8
wal_write_backend | 20960
wal_write_walwriter | 46
wal_write_time | 51
wal_sync_backend | 7
wal_sync_walwriter | 8
wal_sync_time | 0
stats_reset | 2020-10-20 11:04:51.307771+09
```
1. Basic statistics of WAL activity
- wal_records: Total number of WAL records generated
- wal_fpi: Total number of WAL full page images generated
- wal_bytes: Total amount of WAL bytes generated
To understand DB's performance, first, we will check the performance
trends for the entire database instance.
For example, if the number of wal_fpi becomes higher, users may tune
"wal_compression", "checkpoint_timeout" and so on.
Although users can check the above statistics via EXPLAIN,
auto_explain,
autovacuum and pg_stat_statements now,
if users want to see the performance trends for the entire
database,
they must recalculate the statistics.
I think it is useful to add the sum of the basic statistics.
2. WAL segment file creation
- wal_init_file: Total number of WAL segment files created.
To create a new WAL file may have an impact on the performance of
a write-heavy workload generating lots of WAL. If this number is
reported high,
to reduce the number of this initialization, we can tune WAL-related
parameters
so that more "recycled" WAL files can be held.
3. Number
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020-11-12 16:27, Fujii Masao wrote:
On 2020/11/12 14:58, Fujii Masao wrote:
On 2020/11/06 10:25, Masahiro Ikeda wrote:
On 2020-10-30 11:50, Fujii Masao wrote:
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both
number
are important.
Also about performance, I thought there are few impacts because
it
increments stats in memory. If I can implement to reuse
pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes -
prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi -
prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes -
prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
Yes, thanks. I fixed it.
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two
for
walwriter and other processes. Instead, we can use one
m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the
counters
per process type if necessary. If we adopt this approach, we can
easily
extend pg_stat_wal so that any fields can be reported per process
type.
I'll remove the above source code because these counters are not
useful.
On 2020-10-30 12:00, Fujii Masao wrote:
On 2020/10/20 11:31, Masahiro Ikeda wrote:
Hi,
I think we need to add some statistics to pg_stat_wal view.
Although there are some parameter related WAL,
there are few statistics for tuning them.
I think it's better to provide the following statistics.
Please let me know your comments.
```
postgres=# SELECT * from pg_stat_wal;
-[ RECORD 1 ]---+--
wal_records | 2000224
wal_fpi | 47
wal_bytes | 248216337
wal_buffers_full | 20954
wal_init_file | 8
wal_write_backend | 20960
wal_write_walwriter | 46
wal_write_time | 51
wal_sync_backend | 7
wal_sync_walwriter | 8
wal_sync_time | 0
stats_reset | 2020-10-20 11:04:51.307771+09
```
1. Basic statistics of WAL activity
- wal_records: Total number of WAL records generated
- wal_fpi: Total number of WAL full page images generated
- wal_bytes: Total amount of WAL bytes generated
To understand DB's performance, first, we will check the
performance
trends for the entire database instance.
For example, if the number of wal_fpi becomes higher, users may
tune
"wal_compression", "checkpoint_timeout" and so on.
Although users can check the above statistics via EXPLAIN,
auto_explain,
autovacuum and pg_stat_statements now,
if users want to see the performance trends for the entire
database,
they must recalculate the statistics.
I think it is useful to add the sum of the basic statistics.
2. WAL segment file creation
- wal_init_file: Total number of WAL segment files created.
To create a new WAL file may have an impact on the performance of
a write-heavy workload generating lots of WAL. If this number is
reported high,
to reduce the number of this initialization, we can tune
WAL-related parameters
so that more
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
>Now, pg_stat_wal supports reset all informantion in WalStats
>using pg_stat_reset_shared('wal') function.
>Isn't it enough?
Yes it ok, sorry I miss this infomation.
>> 3. I do not think it's a correct describe in document for
>> 'wal_buffers_full'.
>Where should I rewrite the description? If my understanding is not
>correct, please let me know.
Sorry I have not described it clearly, because I can not understand the meaning
of this
column after I read the describe in document.
And now I read the source code of walwrite and found the place where
'wal_buffers_full'
added is for a backend to wait a wal buffer which is occupied by other wal
page, so the
backend flush the old page in the wal buffer(after wait it can).
So i think the origin decribe in document is not so in point, we can describe
it such as
'Total number of times WAL data written to the disk because a backend yelled a
wal buffer
for an advanced wal page.
Sorry if my understand is wrong.
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020/11/12 14:58, Fujii Masao wrote:
On 2020/11/06 10:25, Masahiro Ikeda wrote:
On 2020-10-30 11:50, Fujii Masao wrote:
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both number
are important.
Also about performance, I thought there are few impacts because it
increments stats in memory. If I can implement to reuse pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
Yes, thanks. I fixed it.
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two for
walwriter and other processes. Instead, we can use one m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the counters
per process type if necessary. If we adopt this approach, we can easily
extend pg_stat_wal so that any fields can be reported per process type.
I'll remove the above source code because these counters are not useful.
On 2020-10-30 12:00, Fujii Masao wrote:
On 2020/10/20 11:31, Masahiro Ikeda wrote:
Hi,
I think we need to add some statistics to pg_stat_wal view.
Although there are some parameter related WAL,
there are few statistics for tuning them.
I think it's better to provide the following statistics.
Please let me know your comments.
```
postgres=# SELECT * from pg_stat_wal;
-[ RECORD 1 ]---+--
wal_records | 2000224
wal_fpi | 47
wal_bytes | 248216337
wal_buffers_full | 20954
wal_init_file | 8
wal_write_backend | 20960
wal_write_walwriter | 46
wal_write_time | 51
wal_sync_backend | 7
wal_sync_walwriter | 8
wal_sync_time | 0
stats_reset | 2020-10-20 11:04:51.307771+09
```
1. Basic statistics of WAL activity
- wal_records: Total number of WAL records generated
- wal_fpi: Total number of WAL full page images generated
- wal_bytes: Total amount of WAL bytes generated
To understand DB's performance, first, we will check the performance
trends for the entire database instance.
For example, if the number of wal_fpi becomes higher, users may tune
"wal_compression", "checkpoint_timeout" and so on.
Although users can check the above statistics via EXPLAIN, auto_explain,
autovacuum and pg_stat_statements now,
if users want to see the performance trends for the entire database,
they must recalculate the statistics.
I think it is useful to add the sum of the basic statistics.
2. WAL segment file creation
- wal_init_file: Total number of WAL segment files created.
To create a new WAL file may have an impact on the performance of
a write-heavy workload generating lots of WAL. If this number is reported high,
to reduce the number of this initialization, we can tune WAL-related parameters
so that more "recycled" WAL files can be held.
3. Number of when WAL is
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020/11/06 10:25, Masahiro Ikeda wrote:
On 2020-10-30 11:50, Fujii Masao wrote:
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both number
are important.
Also about performance, I thought there are few impacts because it
increments stats in memory. If I can implement to reuse pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
Yes, thanks. I fixed it.
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two for
walwriter and other processes. Instead, we can use one m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the counters
per process type if necessary. If we adopt this approach, we can easily
extend pg_stat_wal so that any fields can be reported per process type.
I'll remove the above source code because these counters are not useful.
On 2020-10-30 12:00, Fujii Masao wrote:
On 2020/10/20 11:31, Masahiro Ikeda wrote:
Hi,
I think we need to add some statistics to pg_stat_wal view.
Although there are some parameter related WAL,
there are few statistics for tuning them.
I think it's better to provide the following statistics.
Please let me know your comments.
```
postgres=# SELECT * from pg_stat_wal;
-[ RECORD 1 ]---+--
wal_records | 2000224
wal_fpi | 47
wal_bytes | 248216337
wal_buffers_full | 20954
wal_init_file | 8
wal_write_backend | 20960
wal_write_walwriter | 46
wal_write_time | 51
wal_sync_backend | 7
wal_sync_walwriter | 8
wal_sync_time | 0
stats_reset | 2020-10-20 11:04:51.307771+09
```
1. Basic statistics of WAL activity
- wal_records: Total number of WAL records generated
- wal_fpi: Total number of WAL full page images generated
- wal_bytes: Total amount of WAL bytes generated
To understand DB's performance, first, we will check the performance
trends for the entire database instance.
For example, if the number of wal_fpi becomes higher, users may tune
"wal_compression", "checkpoint_timeout" and so on.
Although users can check the above statistics via EXPLAIN, auto_explain,
autovacuum and pg_stat_statements now,
if users want to see the performance trends for the entire database,
they must recalculate the statistics.
I think it is useful to add the sum of the basic statistics.
2. WAL segment file creation
- wal_init_file: Total number of WAL segment files created.
To create a new WAL file may have an impact on the performance of
a write-heavy workload generating lots of WAL. If this number is reported high,
to reduce the number of this initialization, we can tune WAL-related parameters
so that more "recycled" WAL files can be held.
3. Number of when WAL is flushed
- wal_write_backend : Total number of
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020-10-30 11:50, Fujii Masao wrote:
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both number
are important.
Also about performance, I thought there are few impacts because it
increments stats in memory. If I can implement to reuse pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes -
prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi -
prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
Yes, thanks. I fixed it.
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two for
walwriter and other processes. Instead, we can use one m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the
counters
per process type if necessary. If we adopt this approach, we can easily
extend pg_stat_wal so that any fields can be reported per process type.
I'll remove the above source code because these counters are not useful.
On 2020-10-30 12:00, Fujii Masao wrote:
On 2020/10/20 11:31, Masahiro Ikeda wrote:
Hi,
I think we need to add some statistics to pg_stat_wal view.
Although there are some parameter related WAL,
there are few statistics for tuning them.
I think it's better to provide the following statistics.
Please let me know your comments.
```
postgres=# SELECT * from pg_stat_wal;
-[ RECORD 1 ]---+--
wal_records | 2000224
wal_fpi | 47
wal_bytes | 248216337
wal_buffers_full | 20954
wal_init_file | 8
wal_write_backend | 20960
wal_write_walwriter | 46
wal_write_time | 51
wal_sync_backend | 7
wal_sync_walwriter | 8
wal_sync_time | 0
stats_reset | 2020-10-20 11:04:51.307771+09
```
1. Basic statistics of WAL activity
- wal_records: Total number of WAL records generated
- wal_fpi: Total number of WAL full page images generated
- wal_bytes: Total amount of WAL bytes generated
To understand DB's performance, first, we will check the performance
trends for the entire database instance.
For example, if the number of wal_fpi becomes higher, users may tune
"wal_compression", "checkpoint_timeout" and so on.
Although users can check the above statistics via EXPLAIN,
auto_explain,
autovacuum and pg_stat_statements now,
if users want to see the performance trends for the entire database,
they must recalculate the statistics.
I think it is useful to add the sum of the basic statistics.
2. WAL segment file creation
- wal_init_file: Total number of WAL segment files created.
To create a new WAL file may have an impact on the performance of
a write-heavy workload generating lots of WAL. If this number is
reported high,
to reduce the number of this initialization, we can tune WAL-related
parameters
so that more "recycled" WAL files can
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020/10/20 11:31, Masahiro Ikeda wrote: Hi, I think we need to add some statistics to pg_stat_wal view. Although there are some parameter related WAL, there are few statistics for tuning them. I think it's better to provide the following statistics. Please let me know your comments. ``` postgres=# SELECT * from pg_stat_wal; -[ RECORD 1 ]---+-- wal_records | 2000224 wal_fpi | 47 wal_bytes | 248216337 wal_buffers_full | 20954 wal_init_file | 8 wal_write_backend | 20960 wal_write_walwriter | 46 wal_write_time | 51 wal_sync_backend | 7 wal_sync_walwriter | 8 wal_sync_time | 0 stats_reset | 2020-10-20 11:04:51.307771+09 ``` 1. Basic statistics of WAL activity - wal_records: Total number of WAL records generated - wal_fpi: Total number of WAL full page images generated - wal_bytes: Total amount of WAL bytes generated To understand DB's performance, first, we will check the performance trends for the entire database instance. For example, if the number of wal_fpi becomes higher, users may tune "wal_compression", "checkpoint_timeout" and so on. Although users can check the above statistics via EXPLAIN, auto_explain, autovacuum and pg_stat_statements now, if users want to see the performance trends for the entire database, they must recalculate the statistics. I think it is useful to add the sum of the basic statistics. 2. WAL segment file creation - wal_init_file: Total number of WAL segment files created. To create a new WAL file may have an impact on the performance of a write-heavy workload generating lots of WAL. If this number is reported high, to reduce the number of this initialization, we can tune WAL-related parameters so that more "recycled" WAL files can be held. 3. Number of when WAL is flushed - wal_write_backend : Total number of WAL data written to the disk by backends - wal_write_walwriter : Total number of WAL data written to the disk by walwriter - wal_sync_backend : Total number of WAL data synced to the disk by backends - wal_sync_walwriter : Total number of WAL data synced to the disk by walwrite I think it's useful for tuning "synchronous_commit" and "commit_delay" for query executions. If the number of WAL is flushed is high, users can know "synchronous_commit" is useful for the workload. I just wonder how useful these counters are. Even without these counters, we already know synchronous_commit=off is likely to cause the better performance (but has the risk of data loss). So ISTM that these counters are not so useful when tuning synchronous_commit. Regards, -- Fujii Masao Advanced Computing Technology Center Research and Development Headquarters NTT DATA CORPORATION
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020/10/29 17:03, Masahiro Ikeda wrote:
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both number
are important.
Also about performance, I thought there are few impacts because it
increments stats in memory. If I can implement to reuse pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
+ /* fill in some values using pgWalUsage */
+ WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
+ WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
+ WalStats.m_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
It's better to use WalUsageAccumDiff() here?
prevWalUsage needs to be initialized with pgWalUsage?
+ if (AmWalWriterProcess()){
+ WalStats.m_wal_write_walwriter++;
+ }
+ else
+ {
+ WalStats.m_wal_write_backend++;
+ }
I think that it's better not to separate m_wal_write_xxx into two for
walwriter and other processes. Instead, we can use one m_wal_write_xxx
counter and make pgstat_send_wal() send also the process type to
the stats collector. Then the stats collector can accumulate the counters
per process type if necessary. If we adopt this approach, we can easily
extend pg_stat_wal so that any fields can be reported per process type.
Regards,
--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
Hi,
Thanks for your comments and advice. I updated the patch.
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both number
are important.
Also about performance, I thought there are few impacts because it
increments stats in memory. If I can implement to reuse pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes -
prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi -
prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for Horiguchi-san's advice, I changed to reuse pgWalUsage
which is already defined and eliminates the extra overhead.
5. I notice some code in issue_xlog_fsync() function to collect sync
info,
a standby may call the issue_xlog_fsync() too, which do not want to
to collect this info. I think this need some change avoid run by
standby side.
IIUC, issue_xlog_fsync is called by wal receiver on standby side.
But it doesn't send collected statistics to the stats collecter.
So, I think it's not necessary to change the code to avoid collecting
the stats on the standby side.
Regards,
--
Masahiro Ikeda
NTT DATA CORPORATIONdiff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index 98e19954..587acd10 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -3447,12 +3447,106 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i
+
+
+ wal_records bigint
+
+
+ Total number of WAL records generated
+
+
+
+
+
+ wal_fpi bigint
+
+
+ Total number of WAL full page images generated
+
+
+
+
+
+ wal_bytes bigint
+
+
+ Total amount of WAL bytes generated
+
+
+
wal_buffers_full bigint
- Number of times WAL data was written to the disk because WAL buffers got full
+ Total number of times WAL data written to the disk because WAL buffers got full
+
+
+
+
+
+ wal_init_file bigint
+
+
+ Total number of WAL file segment created
+
+
+
+
+
+ wal_write_backend bigint
+
+
+ Total number of times backends write WAL data to the disk
+
+
+
+
+
+ wal_write_walwriter bigint
+
+
+ Total number of times walwriter writes WAL data to the disk
+
+
+
+
+
+ wal_write_time bigint
+
+
+ Total amount of time that has been spent in the portion of
+ WAL data was written to disk by backend and walwriter, in milliseconds
+ (if is enabled, otherwise zero)
+
+
+
+
+
+ wal_sync_backend bigint
+
+
+ Total number of times backends sync WAL data to the disk
+
+
+
+
+
+ wal_sync_walwriter bigint
+
+
+ Total number of times walwriter syncs WAL data synced to the disk
+
+
+
+
+
+ wal_sync_time bigint
+
+
+ Total amount of time that has been spent in the portion of
+ WAL data was synced to disk by backend and walwriter, in milliseconds
+ (if is enabled, otherwise zero)
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 52a67b11..77fe977c 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -2527,6 +2527,8 @@ XLogWrite(XLogwrtRqst WriteRqst, bool flexible)
Size nbytes;
Size nleft;
int written;
+ instr_time start;
+ instr_time duration;
/* OK to write the page(s) */
from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
@@ -2535,9 +2537,28 @@ XLogWrite(XLogwrtRqst WriteRqst,
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
At Thu, 22 Oct 2020 10:44:53 +0900, Masahiro Ikeda
wrote in
> On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
> > At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
> > wrote in
> >> On 2020-10-20 12:46, Amit Kapila wrote:
> >> > I see that we also need to add extra code to capture these stats (some
> >> > of which is in performance-critical path especially in
> >> > XLogInsertRecord) which again makes me a bit uncomfortable. It might
> >> > be that it is all fine as it is very important to collect these stats
> >> > at cluster-level in spite that the same information can be gathered at
> >> > statement-level to help customers but I don't see a very strong case
> >> > for that in your proposal.
> > We should avoid that duplication as possible even if the both number
> > are important.
> >
> >> Also about performance, I thought there are few impacts because it
> >> increments stats in memory. If I can implement to reuse pgWalUsage's
> >> value which already collects these stats, there is no impact in
> >> XLogInsertRecord.
> >> For example, how about pg_stat_wal() calculates the accumulated
> >> value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
> >> value?
> > I don't think that works, but it would work that pgstat_send_wal()
> > takes the difference of that values between two successive calls.
> > WalUsage prevWalUsage;
> > ...
> > pgstat_send_wal()
> > {
> > ..
> >/* fill in some values using pgWalUsage */
> >WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
> >WalStats.m_wal_records = pgWalUsage.wal_records -
> >prevWalUsage.wal_records;
> >WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
> > ...
> >pgstat_send(, sizeof(WalStats));
> >/* remember the current numbers */
> >prevWalUsage = pgWalUsage;
>
> Thanks for your advice. This code can avoid the performance impact of
> critical code.
>
> By the way, what do you think to add these statistics to the
> pg_stat_wal view?
> I thought to remove the above statistics because there is advice that
> PG13's features,
> for example, pg_stat_statement view, vacuum log, and so on can cover
> use-cases.
At Thu, 22 Oct 2020 10:09:21 +0900, Masahiro Ikeda
wrote in
> >> I agreed that the statement-level stat is important and I understood
> >> that we can
> >> know the aggregated WAL stats of pg_stat_statement view and
> >> autovacuum's
> >> log.
> >> But now, WAL stats generated by autovacuum can be output to logs and
> >> it
> >> is not
> >> easy to aggregate them. Since WAL writes impacts for the entire
> >> cluster,
> >> I thought
> >> it's natural to provide accumulated value.
> >>
> > I think it is other way i.e if we would have accumulated stats then it
> > makes sense to provide those at statement-level because one would like
> > to know the exact cause of more WAL activity. Say it is due to an
> > autovacuum or due to the particular set of statements then it would
> > easier for users to do something about it.
>
> OK, I'll remove them.
> Do you have any comments for other statistics?
That discussion comes from the fact that the code adds duplicate code
in a hot path. If we that extra cost doesn't exist, we are free to
add the accumulated values in pg_stat_wal. I think they are useful for
stats-collecting tools as far as we can do that without such an extra
cost.
regards.
--
Kyotaro Horiguchi
NTT Open Source Software Center
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020-10-21 18:03, Kyotaro Horiguchi wrote:
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
On 2020-10-20 12:46, Amit Kapila wrote:
> I see that we also need to add extra code to capture these stats (some
> of which is in performance-critical path especially in
> XLogInsertRecord) which again makes me a bit uncomfortable. It might
> be that it is all fine as it is very important to collect these stats
> at cluster-level in spite that the same information can be gathered at
> statement-level to help customers but I don't see a very strong case
> for that in your proposal.
We should avoid that duplication as possible even if the both number
are important.
Also about performance, I thought there are few impacts because it
increments stats in memory. If I can implement to reuse pgWalUsage's
value which already collects these stats, there is no impact in
XLogInsertRecord.
For example, how about pg_stat_wal() calculates the accumulated
value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes -
prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records -
prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi -
prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
Thanks for your advice. This code can avoid the performance impact of
critical code.
By the way, what do you think to add these statistics to the pg_stat_wal
view?
I thought to remove the above statistics because there is advice that
PG13's features,
for example, pg_stat_statement view, vacuum log, and so on can cover
use-cases.
regards,
--
Masahiro Ikeda
NTT DATA CORPORATION
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020-10-21 15:54, lchch1...@sina.cn wrote:
I think it's really a more convenient way to collect wal usage
information,
with it we can query when I want. Several points on my side:
Thanks for your comments.
1. It will be nice If you provide a chance to reset the information in
WalStats,
so that we can reset it without restart the database.
I agree.
Now, pg_stat_wal supports reset all informantion in WalStats
using pg_stat_reset_shared('wal') function.
Isn't it enough?
2. I think 'wal_write_backend' is mean wal write times happen in
backends. The describe in document is not so clear, suggest rewrite
it.
OK, I'll rewrite to "Total number of times backends write WAL data to
the disk".
3. I do not think it's a correct describe in document for
'wal_buffers_full'.
Where should I rewrite the description? If my understanding is not
correct, please let me know.
4. Quite strange to collect twice in XLogInsertRecord() for
xl_tot_len,
m_wal_records, m_wal_fpi.
Yes, Amit-san pointed me that too.
I'll remove them from pg_stat_wal since pg_stat_statements and vacuum
log
already shows the related statistics and there is a comment it's enough.
Anyway, if you need to the accumulated above statistics in pg_stat_wal,
please let me know.
5. I notice some code in issue_xlog_fsync() function to collect sync
info,
a standby may call the issue_xlog_fsync() too, which do not want to
to collect this info. I think this need some change avoid run by
standby
side.
Thanks, I will fix it.
Regards,
--
Masahiro Ikeda
NTT DATA CORPORATION
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020-10-21 13:41, Amit Kapila wrote: On Tue, Oct 20, 2020 at 12:41 PM Masahiro Ikeda wrote: On 2020-10-20 12:46, Amit Kapila wrote: > On Tue, Oct 20, 2020 at 8:01 AM Masahiro Ikeda >> 1. Basic statistics of WAL activity >> >> - wal_records: Total number of WAL records generated >> - wal_fpi: Total number of WAL full page images generated >> - wal_bytes: Total amount of WAL bytes generated >> >> To understand DB's performance, first, we will check the performance >> trends for the entire database instance. >> For example, if the number of wal_fpi becomes higher, users may tune >> "wal_compression", "checkpoint_timeout" and so on. >> >> Although users can check the above statistics via EXPLAIN, >> auto_explain, >> autovacuum and pg_stat_statements now, >> if users want to see the performance trends for the entire database, >> they must recalculate the statistics. >> > > Here, do you mean to say 'entire cluster' instead of 'entire database' > because it seems these stats are getting collected for the entire > cluster? Thanks for your comments. Yes, I wanted to say 'entire cluster'. >> I think it is useful to add the sum of the basic statistics. >> > > There is an argument that it is better to view these stats at the > statement-level so that one can know which statements are causing most > WAL and then try to rate-limit them if required in the application and > anyway they can get the aggregate of all the WAL if they want. We have > added these stats in PG-13, so do we have any evidence that the > already added stats don't provide enough information? I understand > that you are trying to display the accumulated stats here which if > required users/DBA need to compute with the currently provided stats. > OTOH, sometimes adding more ways to do some things causes difficulty > for users to understand and learn. I agreed that the statement-level stat is important and I understood that we can know the aggregated WAL stats of pg_stat_statement view and autovacuum's log. But now, WAL stats generated by autovacuum can be output to logs and it is not easy to aggregate them. Since WAL writes impacts for the entire cluster, I thought it's natural to provide accumulated value. I think it is other way i.e if we would have accumulated stats then it makes sense to provide those at statement-level because one would like to know the exact cause of more WAL activity. Say it is due to an autovacuum or due to the particular set of statements then it would easier for users to do something about it. OK, I'll remove them. Do you have any comments for other statistics? -- Masahiro Ikeda NTT DATA CORPORATION
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda
wrote in
> On 2020-10-20 12:46, Amit Kapila wrote:
> > I see that we also need to add extra code to capture these stats (some
> > of which is in performance-critical path especially in
> > XLogInsertRecord) which again makes me a bit uncomfortable. It might
> > be that it is all fine as it is very important to collect these stats
> > at cluster-level in spite that the same information can be gathered at
> > statement-level to help customers but I don't see a very strong case
> > for that in your proposal.
We should avoid that duplication as possible even if the both number
are important.
> Also about performance, I thought there are few impacts because it
> increments stats in memory. If I can implement to reuse pgWalUsage's
> value which already collects these stats, there is no impact in
> XLogInsertRecord.
> For example, how about pg_stat_wal() calculates the accumulated
> value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
> value?
I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.
WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
...
pgstat_send(, sizeof(WalStats));
/* remember the current numbers */
prevWalUsage = pgWalUsage;
regards.
--
Kyotaro Horiguchi
NTT Open Source Software Center
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
I think it's really a more convenient way to collect wal usage information, with it we can query when I want. Several points on my side: 1. It will be nice If you provide a chance to reset the information in WalStats, so that we can reset it without restart the database. 2. I think 'wal_write_backend' is mean wal write times happen in backends. The describe in document is not so clear, suggest rewrite it. 3. I do not think it's a correct describe in document for 'wal_buffers_full'. 4. Quite strange to collect twice in XLogInsertRecord() for xl_tot_len, m_wal_records, m_wal_fpi. 5. I notice some code in issue_xlog_fsync() function to collect sync info, a standby may call the issue_xlog_fsync() too, which do not want to to collect this info. I think this need some change avoid run by standby side. Regards, Highgo Software (Canada/China/Pakistan) URL : www.highgo.ca EMAIL: mailto:movead(dot)li(at)highgo(dot)ca
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On Tue, Oct 20, 2020 at 12:41 PM Masahiro Ikeda wrote: > > On 2020-10-20 12:46, Amit Kapila wrote: > > On Tue, Oct 20, 2020 at 8:01 AM Masahiro Ikeda > >> 1. Basic statistics of WAL activity > >> > >> - wal_records: Total number of WAL records generated > >> - wal_fpi: Total number of WAL full page images generated > >> - wal_bytes: Total amount of WAL bytes generated > >> > >> To understand DB's performance, first, we will check the performance > >> trends for the entire database instance. > >> For example, if the number of wal_fpi becomes higher, users may tune > >> "wal_compression", "checkpoint_timeout" and so on. > >> > >> Although users can check the above statistics via EXPLAIN, > >> auto_explain, > >> autovacuum and pg_stat_statements now, > >> if users want to see the performance trends for the entire database, > >> they must recalculate the statistics. > >> > > > > Here, do you mean to say 'entire cluster' instead of 'entire database' > > because it seems these stats are getting collected for the entire > > cluster? > > Thanks for your comments. > Yes, I wanted to say 'entire cluster'. > > >> I think it is useful to add the sum of the basic statistics. > >> > > > > There is an argument that it is better to view these stats at the > > statement-level so that one can know which statements are causing most > > WAL and then try to rate-limit them if required in the application and > > anyway they can get the aggregate of all the WAL if they want. We have > > added these stats in PG-13, so do we have any evidence that the > > already added stats don't provide enough information? I understand > > that you are trying to display the accumulated stats here which if > > required users/DBA need to compute with the currently provided stats. > > OTOH, sometimes adding more ways to do some things causes difficulty > > for users to understand and learn. > > I agreed that the statement-level stat is important and I understood > that we can > know the aggregated WAL stats of pg_stat_statement view and autovacuum's > log. > But now, WAL stats generated by autovacuum can be output to logs and it > is not > easy to aggregate them. Since WAL writes impacts for the entire cluster, > I thought > it's natural to provide accumulated value. > I think it is other way i.e if we would have accumulated stats then it makes sense to provide those at statement-level because one would like to know the exact cause of more WAL activity. Say it is due to an autovacuum or due to the particular set of statements then it would easier for users to do something about it. -- With Regards, Amit Kapila.
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On 2020-10-20 12:46, Amit Kapila wrote: On Tue, Oct 20, 2020 at 8:01 AM Masahiro Ikeda wrote: Hi, I think we need to add some statistics to pg_stat_wal view. Although there are some parameter related WAL, there are few statistics for tuning them. I think it's better to provide the following statistics. Please let me know your comments. ``` postgres=# SELECT * from pg_stat_wal; -[ RECORD 1 ]---+-- wal_records | 2000224 wal_fpi | 47 wal_bytes | 248216337 wal_buffers_full| 20954 wal_init_file | 8 wal_write_backend | 20960 wal_write_walwriter | 46 wal_write_time | 51 wal_sync_backend| 7 wal_sync_walwriter | 8 wal_sync_time | 0 stats_reset | 2020-10-20 11:04:51.307771+09 ``` 1. Basic statistics of WAL activity - wal_records: Total number of WAL records generated - wal_fpi: Total number of WAL full page images generated - wal_bytes: Total amount of WAL bytes generated To understand DB's performance, first, we will check the performance trends for the entire database instance. For example, if the number of wal_fpi becomes higher, users may tune "wal_compression", "checkpoint_timeout" and so on. Although users can check the above statistics via EXPLAIN, auto_explain, autovacuum and pg_stat_statements now, if users want to see the performance trends for the entire database, they must recalculate the statistics. Here, do you mean to say 'entire cluster' instead of 'entire database' because it seems these stats are getting collected for the entire cluster? Thanks for your comments. Yes, I wanted to say 'entire cluster'. I think it is useful to add the sum of the basic statistics. There is an argument that it is better to view these stats at the statement-level so that one can know which statements are causing most WAL and then try to rate-limit them if required in the application and anyway they can get the aggregate of all the WAL if they want. We have added these stats in PG-13, so do we have any evidence that the already added stats don't provide enough information? I understand that you are trying to display the accumulated stats here which if required users/DBA need to compute with the currently provided stats. OTOH, sometimes adding more ways to do some things causes difficulty for users to understand and learn. I agreed that the statement-level stat is important and I understood that we can know the aggregated WAL stats of pg_stat_statement view and autovacuum's log. But now, WAL stats generated by autovacuum can be output to logs and it is not easy to aggregate them. Since WAL writes impacts for the entire cluster, I thought it's natural to provide accumulated value. I see that we also need to add extra code to capture these stats (some of which is in performance-critical path especially in XLogInsertRecord) which again makes me a bit uncomfortable. It might be that it is all fine as it is very important to collect these stats at cluster-level in spite that the same information can be gathered at statement-level to help customers but I don't see a very strong case for that in your proposal. Also about performance, I thought there are few impacts because it increments stats in memory. If I can implement to reuse pgWalUsage's value which already collects these stats, there is no impact in XLogInsertRecord. For example, how about pg_stat_wal() calculates the accumulated value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's value? Regards -- Masahiro Ikeda NTT DATA CORPORATION
Re: Add statistics to pg_stat_wal view for wal related parameter tuning
On Tue, Oct 20, 2020 at 8:01 AM Masahiro Ikeda wrote: > > Hi, > > I think we need to add some statistics to pg_stat_wal view. > > Although there are some parameter related WAL, > there are few statistics for tuning them. > > I think it's better to provide the following statistics. > Please let me know your comments. > > ``` > postgres=# SELECT * from pg_stat_wal; > -[ RECORD 1 ]---+-- > wal_records | 2000224 > wal_fpi | 47 > wal_bytes | 248216337 > wal_buffers_full| 20954 > wal_init_file | 8 > wal_write_backend | 20960 > wal_write_walwriter | 46 > wal_write_time | 51 > wal_sync_backend| 7 > wal_sync_walwriter | 8 > wal_sync_time | 0 > stats_reset | 2020-10-20 11:04:51.307771+09 > ``` > > 1. Basic statistics of WAL activity > > - wal_records: Total number of WAL records generated > - wal_fpi: Total number of WAL full page images generated > - wal_bytes: Total amount of WAL bytes generated > > To understand DB's performance, first, we will check the performance > trends for the entire database instance. > For example, if the number of wal_fpi becomes higher, users may tune > "wal_compression", "checkpoint_timeout" and so on. > > Although users can check the above statistics via EXPLAIN, auto_explain, > autovacuum and pg_stat_statements now, > if users want to see the performance trends for the entire database, > they must recalculate the statistics. > Here, do you mean to say 'entire cluster' instead of 'entire database' because it seems these stats are getting collected for the entire cluster? > I think it is useful to add the sum of the basic statistics. > There is an argument that it is better to view these stats at the statement-level so that one can know which statements are causing most WAL and then try to rate-limit them if required in the application and anyway they can get the aggregate of all the WAL if they want. We have added these stats in PG-13, so do we have any evidence that the already added stats don't provide enough information? I understand that you are trying to display the accumulated stats here which if required users/DBA need to compute with the currently provided stats. OTOH, sometimes adding more ways to do some things causes difficulty for users to understand and learn. I see that we also need to add extra code to capture these stats (some of which is in performance-critical path especially in XLogInsertRecord) which again makes me a bit uncomfortable. It might be that it is all fine as it is very important to collect these stats at cluster-level in spite that the same information can be gathered at statement-level to help customers but I don't see a very strong case for that in your proposal. -- With Regards, Amit Kapila.
Add statistics to pg_stat_wal view for wal related parameter tuning
Hi, I think we need to add some statistics to pg_stat_wal view. Although there are some parameter related WAL, there are few statistics for tuning them. I think it's better to provide the following statistics. Please let me know your comments. ``` postgres=# SELECT * from pg_stat_wal; -[ RECORD 1 ]---+-- wal_records | 2000224 wal_fpi | 47 wal_bytes | 248216337 wal_buffers_full| 20954 wal_init_file | 8 wal_write_backend | 20960 wal_write_walwriter | 46 wal_write_time | 51 wal_sync_backend| 7 wal_sync_walwriter | 8 wal_sync_time | 0 stats_reset | 2020-10-20 11:04:51.307771+09 ``` 1. Basic statistics of WAL activity - wal_records: Total number of WAL records generated - wal_fpi: Total number of WAL full page images generated - wal_bytes: Total amount of WAL bytes generated To understand DB's performance, first, we will check the performance trends for the entire database instance. For example, if the number of wal_fpi becomes higher, users may tune "wal_compression", "checkpoint_timeout" and so on. Although users can check the above statistics via EXPLAIN, auto_explain, autovacuum and pg_stat_statements now, if users want to see the performance trends for the entire database, they must recalculate the statistics. I think it is useful to add the sum of the basic statistics. 2. WAL segment file creation - wal_init_file: Total number of WAL segment files created. To create a new WAL file may have an impact on the performance of a write-heavy workload generating lots of WAL. If this number is reported high, to reduce the number of this initialization, we can tune WAL-related parameters so that more "recycled" WAL files can be held. 3. Number of when WAL is flushed - wal_write_backend : Total number of WAL data written to the disk by backends - wal_write_walwriter : Total number of WAL data written to the disk by walwriter - wal_sync_backend : Total number of WAL data synced to the disk by backends - wal_sync_walwriter : Total number of WAL data synced to the disk by walwrite I think it's useful for tuning "synchronous_commit" and "commit_delay" for query executions. If the number of WAL is flushed is high, users can know "synchronous_commit" is useful for the workload. Also, it's useful for tuning "wal_writer_delay" and "wal_writer_flush_after" for wal writer. If the number is high, users can change the parameter for performance. 4. Wait time when WAL is flushed - wal_write_time : Total amount of time that has been spent in the portion of WAL data was written to disk by backend and walwriter, in milliseconds (if track-io-timing is enabled, otherwise zero.) - wal_sync_time : Total amount of time that has been spent in the portion of WAL data was synced to disk by backend and walwriter, in milliseconds (if track-io-timing is enabled, otherwise zero.) If the time becomes much higher, users can detect the possibility of disk failure. Since users can see how much flush time occupies of the query execution time, it may lead to query tuning and so on. Best Regards, -- Masahiro Ikeda NTT DATA CORPORATIONdiff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml index 66566765..075156f8 100644 --- a/doc/src/sgml/monitoring.sgml +++ b/doc/src/sgml/monitoring.sgml @@ -3406,12 +3406,106 @@ SELECT pid, wait_event_type, wait_event FROM pg_stat_activity WHERE wait_event i + + + wal_records bigint + + + Total number of WAL records generated + + + + + + wal_fpi bigint + + + Total number of WAL full page images generated + + + + + + wal_bytes bigint + + + Total amount of WAL bytes generated + + + wal_buffers_full bigint - Number of times WAL data was written to the disk because WAL buffers got full + Total number of WAL data written to the disk because WAL buffers got full + + + + + + wal_init_file bigint + + + Total number of WAL file segment created + + + + + + wal_write_backend bigint + + + Total number of WAL data written to the disk by backends + + + + + + wal_write_walwriter bigint + + + Total number of WAL data written to the disk by walwriter + + + + + + wal_write_time bigint + + + Total amount of time that has been spent in the portion of + WAL data was written to disk by backend and walwriter, in milliseconds + (if is enabled, otherwise zero) + + + + + + wal_sync_backend bigint + + + Total
