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Message-ID: <dcc563d10809161150g76cb96eev6cd78d2618ca64d7@mail.gmail.com>
Date: Tue, 16 Sep 2008 12:50:28 -0600
From: "Scott Marlowe" <scott.marlowe@gmail.com>
To: "kevin kempter" <kevin@kevinkempterllc.com>
Subject: Re: Setting Effective Cache Size
Cc: pgsql-admin@postgresql.org
In-Reply-To: <D9DB4B8A-76A2-41BD-A24E-6C96177CEB24@kevinkempterllc.com>
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References: <D9DB4B8A-76A2-41BD-A24E-6C96177CEB24@kevinkempterllc.com>

On Tue, Sep 16, 2008 at 9:30 AM, kevin kempter
<kevin@kevinkempterllc.com> wrote:
> Hi All;
>
> Is there an easy way to determine the actual OS disk cache size or at least
> a set of guidelines based on the OS and disk subsystem type ?

On a DB only machine, you can expect the OS to use most of the spare
memory for disk cache eventually.  So, if you've got a 16Gig machine
and pgsql and the OS are using ~ 2 Gigs or so, the effective cache
size will be what's left.

OTOH, if the machine does other things (file / print server, web
server, etc...) then you kinda have to fudge factor it.  Generally
pgsql will still use more disk cache than those other things, due to
having more disk access going on all the time, but really it depends
on your usage patterns.

Luckily Effective cache size is a big course knob so being off by a
gig or three isn't a really big deal.