From: Scott Marlowe <scott.marlowe@gmail.com>
To: Gauthier, Dave <dave.gauthier@intel.com>
Cc: Siddharth Shah <siddharth.shah@elitecore.com>
Cc: pgsql-general@postgresql.org <pgsql-general@postgresql.org>
Subject: Re: Shared Buffers
Date: Tue, 3 Mar 2009 23:51:01 -0700
Message-ID: <dcc563d10903032251y54a74c3cmaeb1f23f1d78d30b@mail.gmail.com> (raw)
In-Reply-To: <482E80323A35A54498B8B70FF2B8798003ED1B9CF0@azsmsx504.amr.corp.intel.com>
References: <49ABDB1F.2020208@elitecore.com>
<482E80323A35A54498B8B70FF2B8798003ED1B9CF0@azsmsx504.amr.corp.intel.com>
On Mon, Mar 2, 2009 at 7:40 AM, Gauthier, Dave <dave.gauthier@intel.com> wrote:
> I believe that op system side buffering can play a role too. I our case,
> the DB server (machine & op sys) caches data that it pulled from disk (not
> necessarily from a DB) and also the disk servers do the same. If a block
> was removed from the DB buffer cache to accommodate more recently requested
> data, but the evicted block is live in memory on the DB server or the disk
> server, it can pull from there instead of performing an expensive disk-IO.
This is very true for certain workloads. If your db is bigger than
memory, and you only work on a tiny bit at a time, the kernel is often
better at caching than pgsql. Run something like pgbench on a machine
with say 4 Gig of memory and a 40 Gig pgbench db dir, and you're
better off with 128M or something for shared_buffers than 1G often
times. Truly random small access, has, for me, worked better with
moderate to smaller shared_buffers. Plus if you need to run a few
memory hog queries, the kernel can flush out some free memory from
cache quickly, but shared_buffers is static. So, the kernel file
cache is self tuning.
But you'll have to test it with your application to really see.
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To: pgsql-general@postgresql.org
Cc: scott.marlowe@gmail.com, dave.gauthier@intel.com, siddharth.shah@elitecore.com
Subject: Re: Shared Buffers
In-Reply-To: <dcc563d10903032251y54a74c3cmaeb1f23f1d78d30b@mail.gmail.com>
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