RE: pg_xlog unbounded growth

Have you tried
archive_command='/bin/true'
as Andreas wrote?

-----Original Message-----
From: Stefan Petrea [mailto:Stefan(dot)Petrea(at)tangoe(dot)com]
Sent: Wednesday, January 24, 2018 2:48 PM
To: pgsql-performance(at)postgresql(dot)org
Subject: pg_xlog unbounded growth

Hello,

This email is structured in sections as follows:

1 - Estimating the size of pg_xlog depending on postgresql.conf parameters
2 - Cleaning up pg_xlog using a watchdog script
3 - Mailing list survey of related bugs
4 - Thoughts

We're using PostgreSQL 9.6.6 on a Ubuntu 16.04.3 LTS.
During some database imports(using pg_restore), we're noticing fast and
unbounded growth of pg_xlog up to the point where the partition(280G in size
for us) that stores it fills up and PostgreSQL shuts down. The error seen in
the logs:

2018-01-17 01:46:23.035 CST [41671] LOG: database system was shut down
at 2018-01-16 15:49:26 CST
2018-01-17 01:46:23.038 CST [41671] FATAL: could not write to file
"pg_xlog/xlogtemp.41671": No space left on device
2018-01-17 01:46:23.039 CST [41662] LOG: startup process (PID 41671)
exited with exit code 1
2018-01-17 01:46:23.039 CST [41662] LOG: aborting startup due to
startup process failure
2018-01-17 01:46:23.078 CST [41662] LOG: database system is shut down

The config settings I thought were relevant are these ones (but I'm also
attaching the entire postgresql.conf if there are other ones that I missed):

wal_level=replica
archive_command='exit 0;'
min_wal_size=2GB
max_wal_size=500MB
checkpoint_completion_target = 0.7
wal_keep_segments = 8

So currently the pg_xlog is growing a lot, and there doesn't seem to be any
way to stop it.

There are some formulas I came across that allow one to compute the maximum
number of WAL allowed in pg_xlog as a function of the PostgreSQL config
parameters.

1.1) Method from 2012 found in [2]

The formula for the upper bound for WAL files in pg_xlog is

(2 + checkpoint_completion_target) * checkpoint_segments + 1 which is ( (2 +
0.7) * (2048/16 * 1/3 ) ) + 1 ~ 116 WAL files

I used the 1/3 because of [6] the shift from checkpoint_segments to
max_wal_size in 9.5 , the relevant quote from the release notes being:

If you previously adjusted checkpoint_segments, the following formula
will give you an approximately equivalent setting:
max_wal_size = (3 * checkpoint_segments) * 16MB

Another way of computing it, also according to [2] is the following
2 * checkpoint_segments + wal_keep_segments + 1 which is (2048/16) + 8 + 1 =
137 WAL files

So far we have two answers, in practice none of them check out, since
pg_xlog grows indefinitely.

1.2) Method from the PostgreSQL internals book

The book [4] says the following:

it could temporarily become up to "3 * checkpoint_segments + 1"

Ok, let's compute this too, it's 3 * (128/3) + 1 = 129 WAL files

This doesn't check out either.

1.3) On the mailing list [3] , I found similar formulas that were seen
previously.

1.4) The post at [5] says max_wal_size is as soft limit and also sets
wal_keep_segments = 0 in order to enforce keeping as little WAL as possible
around. Would this work?

Does wal_keep_segments = 0 turn off WAL recycling? Frankly, I would rather
have WAL not be recycled/reused, and just deleted to keep pg_xlog below
expected size.

Another question is, does wal_level = replica affect the size of pg_xlog in
any way? We have an archive_command that just exits with exit code 0, so I
don't see any reason for the pg_xlog files to not be cleaned up.

2) Cleaning up pg_xlog using a watchdog script

To get the import done I wrote a script that's actually inspired from a blog
post where the pg_xlog out of disk space problem is addressed [1]. It
periodically reads the last checkpoint's REDO WAL file, and deletes all WAL
in pg_xlog before that one.

The intended usage is for this script to run alongside the imports in order
for pg_xlog to be cleaned up gradually and prevent the disk from filling up.

Unlike the blog post and probably slightly wrong is that I used
lexicographic ordering and not ordering by date. But I guess it worked
because the checks were frequent enough that no WAL ever got recycled. In
retrospect I should've used the date ordering.

Does this script have the same effect as checkpoint_completion_target=0 ?

At the end of the day, this script seems to have allowed the import we
needed to get done, but I acknowledge it was a stop-gap measure and not a
long-term solution, hence me posting on the mailing list to find a better
solution.

3) Mailing list survey of related bugs

On the mailing lists, in the past, there have been bugs around pg_xlog
growing out of control:

BUG 7902 [7] - Discusses a situation where WAL are produced faster than
checkpoints can be completed(written to disk), and therefore the WALs in
pg_xlog cannot be recycled/deleted. The status of this bug report is
unclear. I have a feeling it's still open. Is that the case?

BUG 14340 [9] - A user(Sonu Gupta) is reporting pg_xlog unbounded growth and
is asked to do some checks and then directed to the pgsql-general mailing
list where he did not follow up.
I quote the checks that were suggested

Check that your archive_command is functioning correctly, and that you
don't have any inactive replication slots (select * from
pg_replication_slots where not active). Also check the server logs if
both those things are okay.

I have done these checks, and the archive_command we have is returning zero
always.
And we do not have inactive replication slots.

BUG 10013 [12] - A user reports initdb to fill up the disk once he changes
BLCKSZ and/or XLOG_BLCKSZ to non-standard values. The bug seems to be open.

BUG 11989 [8] - A user reports a pg_xlog unbounded growth that concludes in
a disk outage. No further replies after the bug report.

BUG 2104 [10] - A user reports a PostgreSQL not recycling pg_xlog files.
It's suggested that this might have happened because checkpoints were
failing so WAL segments could not be recycled.

BUG 7801 [11] - This is a bit offtopic for our problem(since we don't have
replication set up yet for the server with unbound pg_xlog growth), but
still an interesting read.

A slave falls too far behind a master which leads to increase of pg_xlog on
the slave. The user says making
checkpoint_completion_target=0 or, manually running CHECKPOINT on the slave
is immediately freeing up space on the slave's pg_xlog.

I also learned here that a CHECKPOINT occurs approximately every
checkpoint_completion_target * checkpoint_timeout. Is this correct?

Should I set checkpoint_completion_target=0?

4) Thoughts

In the logs, there are lines like the following one:

28 2018-01-17 02:34:39.407 CST [59922] HINT: Consider increasing the
configuration parameter "max_wal_size".
29 2018-01-17 02:35:02.513 CST [59922] LOG: checkpoints are occurring
too frequently (23 seconds apart)

This looks very similar to BUG 7902 [7]. Is there any rule of thumb,
guideline or technique that can be used when checkpoints cannot be completed
fast enough ?

I'm not sure if this is a misconfiguration problem or a bug. Which one would
be more appropriate?

Thanks,
Stefan

[1]
https://www.endpoint.com/blog/2014/09/25/pgxlog-disk-space-problem-on-postgr
es
[2]
http://chirupgadmin.blogspot.ro/2012/02/wal-growth-calculation-pgxlog-direct
ory.html
[3]
https://www.postgresql.org/message-id/AANLkTi=e=oR54OuxAw88=dtV4wt0e5edMiGae
ZtBVcKO(at)mail(dot)gmail(dot)com
[4] http://www.interdb.jp/blog/pgsql/pg95walsegments/
[5]
http://liufuyang.github.io/2017/09/26/postgres-cannot-auto-clean-up-folder-p
g_xlog.html
[6] https://www.postgresql.org/docs/9.5/static/release-9-5.html#AEN128150
[7]
https://www.postgresql.org/message-id/flat/E1U91WW-0006rq-82%40wrigleys.post
gresql.org
[8]
https://www.postgresql.org/message-id/20141117190201.2478.7245@wrigleys.post
gresql.org
[9]
https://www.postgresql.org/message-id/flat/8a3a6780-18f6-d23a-2350-ac7ad335c
9e7%402ndquadrant.fr
[10]
https://www.postgresql.org/message-id/flat/20051209134337.94B0BF0BAB%40svr2.
postgresql.org
[11]
https://www.postgresql.org/message-id/flat/E1TsemH-0004dK-KN%40wrigleys.post
gresql.org
[12]
https://www.postgresql.org/message-id/flat/20140414014442.15385.74268%40wrig
leys.postgresql.org

Stefan Petrea
System Engineer, Network Engineering

stefan(dot)petrea(at)tangoe(dot)com
tangoe.com

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