pg_xlog unbounded growth - Mailing list pgsql-performance

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-postgres
[2] http://chirupgadmin.blogspot.ro/2012/02/wal-growth-calculation-pgxlog-directory.html
[3] https://www.postgresql.org/message-id/AANLkTi=e=oR54OuxAw88=dtV4wt0e5edMiGaeZtBVcKO@mail.gmail.com
[4] http://www.interdb.jp/blog/pgsql/pg95walsegments/
[5] http://liufuyang.github.io/2017/09/26/postgres-cannot-auto-clean-up-folder-pg_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.postgresql.org
[8] https://www.postgresql.org/message-id/20141117190201.2478.7245@wrigleys.postgresql.org
[9] https://www.postgresql.org/message-id/flat/8a3a6780-18f6-d23a-2350-ac7ad335c9e7%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.postgresql.org
[12] https://www.postgresql.org/message-id/flat/20140414014442.15385.74268%40wrigleys.postgresql.org

Stefan Petrea
System Engineer, Network Engineering


stefan.petrea@tangoe.com
tangoe.com

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