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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
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doc/src/sgml/config.sgml

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@@ -2146,8 +2146,11 @@ include_dir 'conf.d'
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There is a separate server
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process called the <firstterm>background writer</firstterm>, whose function
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is to issue writes of <quote>dirty</quote> (new or modified) shared
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buffers. It writes shared buffers so server processes handling
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user queries seldom or never need to wait for a write to occur.
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buffers. When the number of clean shared buffers appears to be
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insufficient, the background writer writes some dirty buffers to the
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file system and marks them as clean. This reduces the likelihood
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that server processes handling user queries will be unable to find
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clean buffers and have to write dirty buffers themselves.
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However, the background writer does cause a net overall
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increase in I/O load, because while a repeatedly-dirtied page might
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otherwise be written only once per checkpoint interval, the

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