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