On 11/27/20, 10:58 AM, "Stephen Frost" <sfrost@snowman.net> wrote:
> If you'd like to show that I'm wrong, and it's entirely possible that I
> am, then retry the above with actual load on the system, and also
> actually look at how much outstanding WAL you end up with given the
> different scenarios which has to be replayed during crash recovery.
I did a little experiment to show the behavior I'm referring to. I
used these settings:
checkpoint_completion_target = 0.9
checkpoint_timeout = 30s
max_wal_size = 20GB
WAL segment size is 64MB
I ran the following pgbench command for a few minutes before each
test:
pgbench postgres -T 3600 -c 64 -j 64 -N
For the first test, I killed Postgres just before an automatic, non-
immediate checkpoint completed.
2020-11-28 00:31:57 UTC::@:[51770]:LOG: checkpoint complete...
2020-11-28 00:32:00 UTC::@:[51770]:LOG: checkpoint starting: time
Killed Postgres at 00:32:26 UTC, 29 seconds after latest
checkpoint completed.
2020-11-28 00:32:42 UTC::@:[77256]:LOG: redo starts at 3CF/FD6B8BD0
2020-11-28 00:32:56 UTC::@:[77256]:LOG: redo done at 3D0/C94D1D00
Recovery took 14 seconds and replayed ~3.2 GB of WAL.
postgres=> SELECT pg_wal_lsn_diff('3D0/C94D1D00', '3CF/FD6B8BD0');
pg_wal_lsn_diff
-----------------
3420557616
(1 row)
For the second test, I killed Postgres just after an automatic, non-
immediate checkpoint completed.
2020-11-28 00:41:26 UTC::@:[77475]:LOG: checkpoint complete...
Killed Postgres at 00:41:26 UTC, just after latest checkpoint
completed.
2020-11-28 00:41:42 UTC::@:[8599]:LOG: redo starts at 3D3/152EDD78
2020-11-28 00:41:49 UTC::@:[8599]:LOG: redo done at 3D3/78358A40
Recovery took 7 seconds and replayed ~1.5 GB of WAL.
postgres=> SELECT pg_wal_lsn_diff('3D3/78358A40', '3D3/152EDD78');
pg_wal_lsn_diff
-----------------
1661381832
(1 row)
Granted, I used a rather aggressive checkpoint_timeout, but I think
this demonstrates that waiting for a non-immediate checkpoint to
complete can lower the amount of WAL needed for recovery, even though
it might not lower it as much as waiting for an immediate checkpoint
would.
Nathan
