Greetings, * Bossart, Nathan (bossa...@amazon.com) wrote: > LGTM. I just have a few small wording suggestions.
Agreed, those looked like good suggestions and so I've incorporated them. Updated patch attached. Thanks! Stephen
From 40a529bc0a129e90c9917c1a3df2297ac7f2e073 Mon Sep 17 00:00:00 2001 From: Stephen Frost <sfr...@snowman.net> Date: Tue, 19 Jan 2021 13:53:34 -0500 Subject: [PATCH] Change checkpoint_completion_target default to 0.9 Common recommendations are that the checkpoint should be spread out as much as possible, provided we avoid having it take too long. This change updates the default to 0.9 (from 0.5) to match that recommendation. There was some debate about possibly removing the option entirely but it seems there may be some corner-cases where having it set much lower to try to force the checkpoint to be as fast as possible could result in fewer periods of time of reduced performance due to kernel flushing. General agreement is that the "spread more" is the preferred approach though and those who need to tune away from that value are much less common. Reviewed-By: Michael Paquier, Peter Eisentraut, Tom Lane, David Steele, Nathan Bossart Discussion: https://postgr.es/m/20201207175329.GM16415%40tamriel.snowman.net --- doc/src/sgml/config.sgml | 12 ++++++-- doc/src/sgml/wal.sgml | 29 ++++++++++++------- src/backend/postmaster/checkpointer.c | 2 +- src/backend/utils/misc/guc.c | 2 +- src/backend/utils/misc/postgresql.conf.sample | 2 +- src/test/recovery/t/015_promotion_pages.pl | 1 - 6 files changed, 30 insertions(+), 18 deletions(-) diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index 5679b40dd5..0d101f65f6 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -3302,9 +3302,15 @@ include_dir 'conf.d' <listitem> <para> Specifies the target of checkpoint completion, as a fraction of - total time between checkpoints. The default is 0.5. - This parameter can only be set in the <filename>postgresql.conf</filename> - file or on the server command line. + total time between checkpoints. The default is 0.9, which spreads the + checkpoint across almost all of the available interval, providing fairly + consistent I/O load while also leaving some time for checkpoint + completion overhead. Reducing this parameter is not recommended because + it causes the checkpoint to complete faster. This results in a higher + rate of I/O during the checkpoint followed by a period of less I/O between + the checkpoint completion and the next scheduled checkpoint. This + parameter can only be set in the <filename>postgresql.conf</filename> file + or on the server command line. </para> </listitem> </varlistentry> diff --git a/doc/src/sgml/wal.sgml b/doc/src/sgml/wal.sgml index ae4a3c1293..7d48f42710 100644 --- a/doc/src/sgml/wal.sgml +++ b/doc/src/sgml/wal.sgml @@ -571,22 +571,29 @@ writing dirty buffers during a checkpoint is spread over a period of time. That period is controlled by <xref linkend="guc-checkpoint-completion-target"/>, which is - given as a fraction of the checkpoint interval. + given as a fraction of the checkpoint interval (configured by using + <varname>checkpoint_timeout</varname>). The I/O rate is adjusted so that the checkpoint finishes when the given fraction of <varname>checkpoint_timeout</varname> seconds have elapsed, or before <varname>max_wal_size</varname> is exceeded, whichever is sooner. - With the default value of 0.5, + With the default value of 0.9, <productname>PostgreSQL</productname> can be expected to complete each checkpoint - in about half the time before the next checkpoint starts. On a system - that's very close to maximum I/O throughput during normal operation, - you might want to increase <varname>checkpoint_completion_target</varname> - to reduce the I/O load from checkpoints. The disadvantage of this is that - prolonging checkpoints affects recovery time, because more WAL segments - will need to be kept around for possible use in recovery. Although - <varname>checkpoint_completion_target</varname> can be set as high as 1.0, - it is best to keep it less than that (perhaps 0.9 at most) since - checkpoints include some other activities besides writing dirty buffers. + a bit before the next scheduled checkpoint (at around 90% of the last checkpoint's + duration). This spreads out the I/O as much as possible so that the checkpoint + I/O load is consistent throughout the checkpoint interval. The disadvantage of + this is that prolonging checkpoints affects recovery time, because more WAL + segments will need to be kept around for possible use in recovery. A user + concerned about the amount of time required to recover might wish to reduce + <varname>checkpoint_timeout</varname> so that checkpoints occur more frequently + but still spread the I/O across the checkpoint interval. Alternatively, + <varname>checkpoint_completion_target</varname> could be reduced, but this would + result in times of more intense I/O (during the checkpoint) and times of less I/O + (after the checkpoint completed but before the next scheduled checkpoint) and + therefore is not recommended. + Although <varname>checkpoint_completion_target</varname> could be set as high as + 1.0, it is typically recommended to set it to no higher than 0.9 (the default) + since checkpoints include some other activities besides writing dirty buffers. A setting of 1.0 is quite likely to result in checkpoints not being completed on time, which would result in performance loss due to unexpected variation in the number of WAL segments needed. diff --git a/src/backend/postmaster/checkpointer.c b/src/backend/postmaster/checkpointer.c index 5907a7befc..e7e6a2a459 100644 --- a/src/backend/postmaster/checkpointer.c +++ b/src/backend/postmaster/checkpointer.c @@ -145,7 +145,7 @@ static CheckpointerShmemStruct *CheckpointerShmem; */ int CheckPointTimeout = 300; int CheckPointWarning = 30; -double CheckPointCompletionTarget = 0.5; +double CheckPointCompletionTarget = 0.9; /* * Private state diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c index 3b36a31a47..5fe7f2fa01 100644 --- a/src/backend/utils/misc/guc.c +++ b/src/backend/utils/misc/guc.c @@ -3727,7 +3727,7 @@ static struct config_real ConfigureNamesReal[] = NULL }, &CheckPointCompletionTarget, - 0.5, 0.0, 1.0, + 0.9, 0.0, 1.0, NULL, NULL, NULL }, diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample index 86425965d0..68f95bdfff 100644 --- a/src/backend/utils/misc/postgresql.conf.sample +++ b/src/backend/utils/misc/postgresql.conf.sample @@ -231,7 +231,7 @@ #checkpoint_timeout = 5min # range 30s-1d #max_wal_size = 1GB #min_wal_size = 80MB -#checkpoint_completion_target = 0.5 # checkpoint target duration, 0.0 - 1.0 +#checkpoint_completion_target = 0.9 # checkpoint target duration, 0.0 - 1.0 #checkpoint_flush_after = 0 # measured in pages, 0 disables #checkpoint_warning = 30s # 0 disables diff --git a/src/test/recovery/t/015_promotion_pages.pl b/src/test/recovery/t/015_promotion_pages.pl index 6fb70b5001..25a9e4764a 100644 --- a/src/test/recovery/t/015_promotion_pages.pl +++ b/src/test/recovery/t/015_promotion_pages.pl @@ -26,7 +26,6 @@ my $bravo = get_new_node('bravo'); $bravo->init_from_backup($alpha, 'bkp', has_streaming => 1); $bravo->append_conf('postgresql.conf', <<EOF); checkpoint_timeout=1h -checkpoint_completion_target=0.9 EOF $bravo->start; -- 2.27.0
signature.asc
Description: PGP signature
