Re: [HACKERS] understanding pg_stat* numbers

[Oleg Bartunov]

Qingqing,
Thanks for the info, I do know all issues you mentioned. I want to know
if there is any possibility to research performance problem in PostgreSQL
without voodoo techiques, at least at the level of pg itself. At this aspect, 
I could answer myself: heap_blks_read is the # of blocks
'ordered' to read from disk and heap_blks_hit is the # of blocks
read from pg buffers.

One mystery remains, why stats show heap_blks_read > 0 for indexed search ?
        select 1 from foo where id=5
I did pg_stat_reset() before run query.
Oleg
On Thu, 31 Mar 2005, Qingqing Zhou wrote:
"Oleg Bartunov" <oleg@sai.msu.su> writes
From my experiments I feel heap_blks_read is the table blocks read from
disk,
well, sort of, and heap_blks_hit - from buffer. At least, I observed when
I
repeat the same query several times (heap_blks_read+heap_blks_hit) doesn't
changed while more blocks come from heap_blks_hit.
PG relies on the OS to schedule the *real* IO you mean. So heap_blks_read
may come from OS kernel buffer, or from disk. You can minimize/disable your
OS file buffers (if it supports this function) or change the mdopen() using
O_DIRECT to make the heap_blks_read approaches the *real* IO if you want -
but for sure this hurts performance.
It's common in db worlds to have IO statistics (timings are not important)
to research algorithms and implementation.
The *real* IO you mean might be an elegant choice in academic, but is a
subjective matter in practice. For example, we can only assume statistically
N random IO costs the same time with another N random IO, because disk
header has its own scheduling logic controlled by the layout of these
blocks, etc, which you can't decide. Not to speak of the IOs in concurrent
environment.  If so, use heap_blks_read/write is already good enough?
Regards,
Qingqing

---------------------------(end of broadcast)---------------------------
TIP 1: subscribe and unsubscribe commands go to [EMAIL PROTECTED]
            Regards,
                Oleg
_____________________________________________________________
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
Sternberg Astronomical Institute, Moscow University (Russia)
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
phone: +007(095)939-16-83, +007(095)939-23-83
---------------------------(end of broadcast)---------------------------
TIP 2: you can get off all lists at once with the unregister command
   (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])