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Re: bgwriter_lru_multiplier blurbs inconsistent

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Alvaro Herrera <alvherre(at)commandprompt(dot)com>
Cc: pgsql-docs(at)postgresql(dot)org, Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: bgwriter_lru_multiplier blurbs inconsistent
Date: 2008-01-20 20:08:40
Message-ID: 11682.1200859720@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-docspgsql-hackers
Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> Is the bgwriter_lru_multiplier parameter a limit on the number to scan
> or to write?  GUC and docs seem to contradict one another.  GUC says

> #: utils/misc/guc.c:1834
> #, fuzzy
> msgid "Background writer multiplier on average buffers to scan per round."

> The docs say
>          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.

> Which one is correct?  Do we need a correction of either?

We multiply the average number of new buffers used per round by the
multiplier, and use that as the target for the number of clean buffers
to have in front of the sweep hand.  Any of these that are dirty will
be written (until we exceed maxpages written).  So it's the number to scan.

I don't find either the docs or the msgid to be wrong, exactly; but if
you have a proposal for better wording, I'm all ears.

			regards, tom lane

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