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  LOG: database system was shut down at 2018-01-16 15:49:26 CST 2018-01-17 01:46:23.038 CST  FATAL: could not write to file "pg_xlog/xlogtemp.41671": No space left on device 2018-01-17 01:46:23.039 CST  LOG: startup process (PID 41671) exited with exit code 1 2018-01-17 01:46:23.039 CST  LOG: aborting startup due to startup process failure 2018-01-17 01:46:23.078 CST  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  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  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  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  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  , I found similar formulas that were seen previously. 1.4) The post at  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 . 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  - 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  - 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  - 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  - A user reports a pg_xlog unbounded growth that concludes in a disk outage. No further replies after the bug report. BUG 2104  - 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  - 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  HINT: Consider increasing the configuration parameter "max_wal_size". 29 2018-01-17 02:35:02.513 CST  LOG: checkpoints are occurring too frequently (23 seconds apart) This looks very similar to BUG 7902 . 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  https://www.endpoint.com/blog/2014/09/25/pgxlog-disk-space-problem-on-postgres  http://chirupgadmin.blogspot.ro/2012/02/wal-growth-calculation-pgxlog-directory.html  https://www.postgresql.org/message-id/AANLkTi=e=oR54OuxAw88=dtv4wt0e5edmigaeztbv...@mail.gmail.com  http://www.interdb.jp/blog/pgsql/pg95walsegments/  http://liufuyang.github.io/2017/09/26/postgres-cannot-auto-clean-up-folder-pg_xlog.html  https://www.postgresql.org/docs/9.5/static/release-9-5.html#AEN128150  https://www.postgresql.org/message-id/flat/E1U91WW-0006rq-82%40wrigleys.postgresql.org  https://www.postgresql.org/message-id/20141117190201.2478.7...@wrigleys.postgresql.org  https://www.postgresql.org/message-id/flat/8a3a6780-18f6-d23a-2350-ac7ad335c9e7%402ndquadrant.fr  https://www.postgresql.org/message-id/flat/20051209134337.94B0BF0BAB%40svr2.postgresql.org  https://www.postgresql.org/message-id/flat/E1TsemH-0004dK-KN%40wrigleys.postgresql.org  https://www.postgresql.org/message-id/flat/20140414014442.15385.74268%40wrigleys.postgresql.org Stefan Petrea System Engineer, Network Engineering stefan.pet...@tangoe.com tangoe.com This e-mail message, including any attachments, is for the sole use of the intended recipient of this message, and may contain information that is confidential or legally protected. 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