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Message-ID: <1d219a6f05081710146acb9d40@mail.gmail.com>
Date: Wed, 17 Aug 2005 13:14:29 -0400
From: Chris Hoover <revoohc@gmail.com>
To: pgsql-performance@postgresql.org
Subject: Tuning Effective Cache Question
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I have some questions about tuning my effective_cache_size

I have a RHEL 2.1 box running with dual Xeon (2.6 GHz I believe and
they have HT on).  The box has 8GB memory.  In my postgresql.conf, I
have set the effective_cache_size =3D 530505 (~4GB).

However, I am noticing on this machine, top is telling me that I have
~3.5GB in the buff and only 3GB in cached.

Here are the exact numbers:
Mem:  7720040K av, 7714364K used,    5676K free,  314816K shrd, 3737540K bu=
ff
Swap: 2096440K av,  119448K used, 1976992K free                 3188192K ca=
ched


1.  Is the configuration that linux is running in hurting PostgreSQL in any=
 way?

2.  Is there a negative impact of the effective_cache_size being
larger than the actual cached memory?

3.  What effect postive or negative does the buff memory have on PostgreSQL=
.\

4.  What exactly is this buff memory used for?  We have had a time
trying to find a good explanation of what it means.

Overall, this system appears to be running fine.  However, I was taken
back when I saw the current memory configuration.  We have been
walking a fine line performance wise, and can quickly become i/o
starved.  So I want to make sure that I am not pushing the db towards
the i/o starved side.

Thanks for any insight,

Chris