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<chapter Id="runtime-config">

<title>Server Configuration</title>

<title>Background Writer</title>

<para>

Beginning in <productname>PostgreSQL</> 8.0, there is a separate server

There is a separate server

process called the <firstterm>background writer</>, whose function

is to issue writes of <quote>dirty</> shared buffers. The intent is

that server processes handling user queries should seldom or never have

to wait for a write to occur, because the background writer will do it.

However there is a net overall

increase in I/O load, becausewherea repeatedly-dirtied page might

before have beenwritten only once per checkpoint interval, the

increase in I/O load, because a repeatedly-dirtied page might

otherwise bewritten only once per checkpoint interval, but the

background writer might write it several times in the same interval.

In most situations a continuous low load is preferable to periodic

spikes, but the parameters discussed in this subsection can be used to

The parameters discussed in this subsection can be used to

tune the behavior for local needs.

</para>

</indexterm>

<listitem>

<para>

Unless limited by <varname>bgwriter_lru_maxpages</>, the number

of dirty buffers written in each round is determined by reference

to the number of new buffers that have been needed by server

processes during recent rounds. This number is multiplied by

<varname>bgwriter_lru_multiplier</> to arrive at the estimate

of the number of buffers that will be needed during the next round.

The number of dirty buffers written in each round is based on the

number of new buffers that have been needed by server processes

during recent rounds. The average recent need is multiplied by

<varname>bgwriter_lru_multiplier</> to arrive at an estimate of the

number of buffers that will be needed during the next round. Dirty

buffers are written until there are that many clean, reusable buffers

available. (However, no more than <varname>bgwriter_lru_maxpages</>

buffers will be written per round.)

Thus, a setting of 1.0 represents a <quote>just in time</> policy

of writing exactly the number of buffers predicted to be needed.

Larger values provide some cushion against spikes in demand,