diff options
| -rw-r--r-- | doc/src/sgml/config.sgml | 12 | ||||
| -rw-r--r-- | doc/src/sgml/wal.sgml | 29 | ||||
| -rw-r--r-- | src/backend/postmaster/checkpointer.c | 2 | ||||
| -rw-r--r-- | src/backend/utils/misc/guc.c | 2 | ||||
| -rw-r--r-- | src/backend/utils/misc/postgresql.conf.sample | 2 | ||||
| -rw-r--r-- | 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 d63aebb2ff1..ddc6d789d8a 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 ae4a3c12931..7d48f427109 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 5907a7befc5..e7e6a2a4594 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 cc0b9f6ad62..0c5dc4d3e84 100644 --- a/src/backend/utils/misc/guc.c +++ b/src/backend/utils/misc/guc.c @@ -3725,7 +3725,7 @@ 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 74b416b74a8..b234a6bfe64 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 6fb70b5001b..25a9e4764a2 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; |