From d661532e27b34e9c89d0700c6ce246731e70072c Mon Sep 17 00:00:00 2001 From: Heikki Linnakangas Date: Mon, 29 Jun 2015 00:09:10 +0300 Subject: Also trigger restartpoints based on max_wal_size on standby. When archive recovery and restartpoints were initially introduced, checkpoint_segments was ignored on the grounds that the files restored from archive don't consume any space in the recovery server. That was changed in later releases, but even then it was arguably a feature rather than a bug, as performing restartpoints as often as checkpoints during normal operation might be excessive, but you might nevertheless not want to waste a lot of space for pre-allocated WAL by setting checkpoint_segments to a high value. But now that we have separate min_wal_size and max_wal_size settings, you can bound WAL usage with max_wal_size, and still avoid consuming excessive space usage by setting min_wal_size to a lower value, so that argument is moot. There are still some issues with actually limiting the space usage to max_wal_size: restartpoints in recovery can only start after seeing the checkpoint record, while a checkpoint starts flushing buffers as soon as the redo-pointer is set. Restartpoint is paced to happen at the same leisurily speed, determined by checkpoint_completion_target, as checkpoints, but because they are started later, max_wal_size can be exceeded by upto one checkpoint cycle's worth of WAL, depending on checkpoint_completion_target. But that seems better than not trying at all, and max_wal_size is a soft limit anyway. The documentation already claimed that max_wal_size is obeyed in recovery, so this just fixes the behaviour to match the docs. However, add some weasel-words there to mention that max_wal_size may well be exceeded by some amount in recovery. --- doc/src/sgml/wal.sgml | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) (limited to 'doc/src') diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml index f4083c3fe57..e3941c9391b 100644 --- a/doc/src/sgml/wal.sgml +++ b/doc/src/sgml/wal.sgml @@ -590,7 +590,11 @@ A restartpoint is triggered when a checkpoint record is reached if at least checkpoint_timeout seconds have passed since the last restartpoint, or if WAL size is about to exceed - max_wal_size. + max_wal_size. However, because of limitations on when a + restartpoint can be performed, max_wal_size is often exceeded + during recovery, by up to one checkpoint cycle's worth of WAL. + (max_wal_size is never a hard limit anyway, so you should + always leave plenty of headroom to avoid running out of disk space.) -- cgit v1.2.3