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doc: update bgwriter description

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This clarifies exactly what the bgwriter does, which should help with
tuning.

Reported-by: Chris Wilson

Discussion: https://postgr.es/m/160399562040.7809.7335281028960123489@wrigleys.postgresql.org

Backpatch-through: 9.5
This commit is contained in:
Bruce Momjian 2020-11-16 13:13:43 -05:00
parent f75a7bb6c2
commit b7fc259323
1 changed files with 5 additions and 2 deletions
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doc/src/sgml/config.sgml
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@ -2093,8 +2093,11 @@ include_dir 'conf.d'
There is a separate server There is a separate server
process called the <firstterm>background writer</firstterm>, whose function process called the <firstterm>background writer</firstterm>, whose function
is to issue writes of <quote>dirty</quote> (new or modified) shared is to issue writes of <quote>dirty</quote> (new or modified) shared
buffers. It writes shared buffers so server processes handling buffers. When the number of clean shared buffers appears to be
user queries seldom or never need to wait for a write to occur. insufficient, the background writer writes some dirty buffers to the
file system and marks them as clean. This reduces the likelihood
that server processes handling user queries will be unable to find
clean buffers and have to write dirty buffers themselves.
However, the background writer does cause a net overall However, the background writer does cause a net overall
increase in I/O load, because while a repeatedly-dirtied page might increase in I/O load, because while a repeatedly-dirtied page might
otherwise be written only once per checkpoint interval, the otherwise be written only once per checkpoint interval, the