Date: Tue, 27 May 2014 10:40:13 -0400
From: Stephen Frost <sfrost@snowman.net>
To: Heikki Linnakangas <hlinnakangas@vmware.com>
Cc: Albe Laurenz <laurenz.albe@wien.gv.at>,
 "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: NFS, file system cache and shared_buffers
Message-ID: <20140527144013.GL2556@tamriel.snowman.net>
References: 
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 <53848490.6060001@vmware.com>
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* Heikki Linnakangas (hlinnakangas@vmware.com) wrote:
> On 05/27/2014 02:06 PM, Albe Laurenz wrote:
> >I just learned that NFS does not use a file system cache on the client s=
ide.
> >
> >On the other hand, PostgreSQL relies on the file system cache for perfor=
mance,
> >because beyond a certain amount of shared_buffers performance will suffe=
r.
> >
> >Together these things seem to indicate that you cannot get good performa=
nce
> >with a large database over NFS since you can leverage memory speed.
> >
> >Now I wonder if there are any remedies (CacheFS?) and what experiences
> >people have made with the performance of large databases over NFS.
>=20
> I have no personal experience with NFS, but sounds like a
> higher-than-usual shared_buffers value would be good.

It would certainly be worthwhile to test it.  In the end you would,
hopefully, end up with a situation where you're maximizing RAM usage-
the NFS server is certainly caching in *its* filesystem cache, while on
the PG server you're getting the benefit of shared_buffers without the
drawback of double-buffering (since you couldn't ever use the NFS
server's memory for shared_buffers anyway).

All that said, there has always been a recommendation of caution around
using NFS as a backing store for PG, or any RDBMS..

	Thanks,

		Stephen

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