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To: Dave Cramer <pg@fastcrypt.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: increasing shared buffers: how much should be removed from OS
	filesystem cache?
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	<3743D7B4-2749-41C3-ADE1-39B86F7C98D4@fastcrypt.com>
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From: Guillaume Cottenceau <gc@mnc.ch>
Date: 04 Sep 2006 14:07:37 +0200
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Dave Cramer <pg 'at' fastcrypt.com> writes:

> Guillaume
> 
> 1G is really not a significant amount of memory these days,

Yeah though we have 2G or 4G of RAM in our servers (and not only
postgres running on it).
 
> That said 6-10% of available memory should be given to an 8.0 or
> older version of postgresql
> 
> Newer versions work better around 25%
> 
> I'm not sure what you mean by mechanically removed from effective_cache

I mean that when you allocate more memory to applications, the
consequence is less memory the OS will be able to use for disk
cache.
 
> effective cache is really a representation of shared buffers plus OS
> cache

Are you sure the shared buffers should be counted in? As I
understand the documentation, they should not (as shared buffers
is allocated memory for the OS, not part of "kernel's disk
cache"):

    Sets the planner's assumption about the effective size of the
    disk cache (that is, the portion of the kernel's disk cache
    that will be used for PostgreSQL data files). This is
    measured in disk pages, which are normally 8192 bytes each.
    The default is 1000.

-- 
Guillaume Cottenceau
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