Hello All
We got to the bottom of this in the end - it was due to a misconfiguration that
resulted in random corruption of WAL archives.
Part of the problem was the use of 'pigz' (multi-threaded gzip tool) for
compression of the WAL files, which in our case led to a file that decompresses
without error into a file of the correct size but where 2/3 of the files were
all '0's. Needless to say I have stopped using pigz!
There were a couple if learnings from the whole exercise: first, during
recovery, corrupted WAL files do not result in error messages describing the
corruption in the log. In fact there is code in xlog.c (XLogPageRead) which
validates the WAL file header, and if it finds any errors, discards them and
tries again. From the log file all you can see is repeated attempts to fetch
the WAL file from alternating sources.
The other wrinkle was the missing timeline history file. Even after fixing the
WAL corruption, and starting over with a fresh base backup, it still hiccupped
about 00000003.history being mysteriously absent from the master. The following
sequence was repeatedly present in the log:
[2018-12-24 21:41:26 AEDT] 16246@ LOG: restored log file
"000000030002CE55000000ED" from archive
[2018-12-24 21:41:31 AEDT] 16246@ LOG: restored log file
"000000030002CE55000000EE" from archive
[2018-12-24 21:41:31 AEDT] 16246@ LOG: restored log file
"000000030002CE55000000EF" from archive
[2018-12-24 21:41:32 AEDT] 16246@ LOG: restored log file
"000000030002CE55000000F0" from archive
[2018-12-24 21:41:33 AEDT] 16246@ LOG: restored log file
"000000030002CE55000000F1" from archive
gzip: /var/backup2/dbs1a/pg_xlog/000000030002CE55000000F2.gz: unexpected end of
file
[2018-12-24 21:41:33 AEDT] 4287@ LOG: fetching timeline history file for
timeline 3 from primary server
[2018-12-24 21:41:33 AEDT] 4287@ FATAL: could not receive timeline history
file from the primary server: ERROR: could not open file
"pg_xlog/00000003.history": No such file or directory
It seems like it reached the end of the WAL archive, then tried to switch to
streaming, but then that failed because the master did not have a copy of this
history file. Then it switched back to the WAL archive, and the cycle continued.
In the end I created a fake history file and put it in the master's pg_xlog
dir. After that the switch to WAL streaming was successful and the standby was
at last up to speed.
Anyway, all is well that ends well!
Cheers
Mike
-----Original Message-----
From: Michael Harris
Sent: Friday, December 14, 2018 6:40 PM
To: pgsql-general@lists.postgresql.org
Subject: Hot Standby Replica Recovery Problem
Hello Experts,
We've been having a strange problem with one of our databases.
A summary of the setup follows:
- We are running postgresql 9.6.9 on Centos 7.
- We are using postgresql native streaming replication
- There is one master and one hot standby
- The master is archiving it's WAL files with this archive command:
archive_command = 'test ! -f %p || (test ! -f
/var/backup2/dbs1a/pg_xlog/%f.gz && pigz -c %p >
/var/backup2/dbs1a/pg_xlog/%f.gz)' # command to use to archive a
logfile segment
/var/backup2 is a shared gfs2 filesystem.
- Recently we had to do some maintenance that involved rebuilding the database
servers.
At that point dbs1b was the master, so we promoted dbs1a, did the work on
dbs1b,
and then began establishing a new replica on dbs1b using pg_basebackup
- After pg_basebackup completed we set up recovery.conf:
restore_command='gzip -dc /var/backup2/dbs1a/pg_xlog/%f.gz > %p'
standby_mode='on'
primary_conninfo='host=dbs1a user=xxxxxx password=xxxxx'
and then started postgresql.
At first it seemed to be starting up as per normal. It began REDO-ing WAL files
from the archive. It reached a certain point and then failed with this:
[2018-12-14 11:07:39 AEDT] 2050@ LOG: restored log file
"000000030002C4D700000028" from archive
[2018-12-14 11:07:39 AEDT] 29240@ LOG: fetching timeline history file for
timeline 3 from primary server
[2018-12-14 11:07:39 AEDT] 29240@ FATAL: could not receive timeline history
file from the primary server: ERROR: could not open file
"pg_xlog/00000003.history": No such file or directory
These messages repeated every few seconds until we stopped the postmaster.
I was surprised because we have done this procedure many times on this & other
databases, but anyway Dr Google suggested this
https://superuser.com/questions/1127360/cant-find-the-timeline-history-file-to-get-the-replication-working.
After reading that and other linked articles I decided to create a 'dummy'
history file.
This time the startup sequence failed like this:
[2018-12-14 16:26:46 AEDT] 16575@ LOG: restored log file
"000000030002C4D700000028" from archive
[2018-12-14 16:26:47 AEDT] 17669@ LOG: fetching timeline history file for
timeline 3 from primary server
[2018-12-14 16:26:47 AEDT] 17669@ LOG: started streaming WAL from primary at
2C4D7/28000000 on timeline 3
[2018-12-14 16:26:47 AEDT] 17669@ FATAL: could not receive data from WAL
stream: ERROR: requested WAL segment 000000030002C4D700000028 has already been
removed
Again I was surprised - why wasn't it getting this file from the archives? It
is present and does not seem to be truncated or corrupted (as far as I can
tell).
Anyway in an attempt to force it to get the file from the archives, I removed
the primary_conninfo.
This time it just repeatedly fetched the file 000000030002C4D700000028 again
and again, never progressing to the next one.
Finally I turned up the debugging output, to see this:
[2018-12-14 17:58:25 AEDT] 12365@ DEBUG: executing restore command "gzip -dc
/var/backup2/dbs1a/pg_xlog/000000030002C4D700000028.gz > pg_xlog/RECOVERYXLOG"
[2018-12-14 17:58:25 AEDT] 12365@ LOG: restored log file
"000000030002C4D700000028" from archive
[2018-12-14 17:58:25 AEDT] 12365@ DEBUG: got WAL segment from archive
[2018-12-14 17:58:25 AEDT] 12365@ DEBUG: switched WAL source from archive to
stream after failure
[2018-12-14 17:58:25 AEDT] 12365@ DEBUG: switched WAL source from stream to
archive after failure
It seems like something is going wrong with the WAL file replay here, although
I cannot see what??
I'm writing this email after the second attempt at replication: the first
attempt ended much the same.
I assumed I had done something wrong so I re-started from the pg_basebackup.
Now I am concerned that something is wrong with the master, or maybe with our
WAL archiving process.
Any tips would be gratefully received!
Regards
Mike
