From 62b50ad698f8a789f92177ad72639f99d94d7d78 Mon Sep 17 00:00:00 2001 From: Bruce Momjian Date: Mon, 16 Nov 2020 13:13:43 -0500 Subject: [PATCH] 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 --- doc/src/sgml/config.sgml | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml index f043433e31..a632cf98ba 100644 --- a/doc/src/sgml/config.sgml +++ b/doc/src/sgml/config.sgml @@ -2146,8 +2146,11 @@ include_dir 'conf.d' There is a separate server process called the background writer, whose function is to issue writes of dirty (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