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doc: update bgwriter description 

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 8ad06bcbe1
commit 62b50ad698
1 changed files with 5 additions and 2 deletions
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7
doc/src/sgml/config.sgml
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@ -2146,8 +2146,11 @@ include_dir 'conf.d'
There is a separate server
process called the <firstterm>background writer</firstterm>, whose function
is to issue writes of <quote>dirty</quote> (new or modified) shared
buffers. It writes shared buffers so server processes handling
user queries seldom or never need to wait for a write to occur.
buffers. When the number of clean shared buffers appears to be
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
increase in I/O load, because while a repeatedly-dirtied page might
otherwise be written only once per checkpoint interval, the