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From: Greg Smith <gsmith@gregsmith.com>
To: Matthew Pulis <mpulis@gmail.com>
Cc: pgsql-admin@postgresql.org
Cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] PostgreSQL Cache
Date: Mon, 29 Sep 2008 02:55:52 -0400 (EDT)
Message-ID: <Pine.GSO.4.64.0809290237330.18983@westnet.com> (raw)
In-Reply-To: <f88da7150809282143w71c0920avb51f7809eb398ef7@mail.gmail.com>
References: <f88da7150809282143w71c0920avb51f7809eb398ef7@mail.gmail.com>

On Mon, 29 Sep 2008, Matthew Pulis wrote:

> I need to perform some timed testing, thus need to make sure that disk cache
> does not affect me. Is clearing the OS (Ubuntu) disk cache, ( by running:
> sudo echo 3 | sudo tee /proc/sys/vm/drop_caches ) enough to do this?

What you should do is:

1) Shutdown the database server (pg_ctl, sudo service postgresql stop, 
etc.)
2) sync
3) sudo echo 3 > /proc/sys/vm/drop_caches
4) Start the database server

That will clear both the database and OS cache with a minimum of junk left 
behind in the process; clearing the cache without a sync is a bad idea.

Note that all of this will still leave behind whatever cache is in your 
disk controller card or on the disk themselves available.  There are some 
other techniques you could consider.  Add a setp 2.5 that generates a 
bunch of data unused by the test, then sync again, and you've turned most 
of that into useless caching.

Ideally, your test should be running against a large enough data set that 
the dozens or couple of hundred megabytes that might be in those will only 
add a bit of noise to whatever you're testing.  If you're not running a 
larger test or going through tasts to make the caches clear, the only easy 
way to make things more clear is to reboot the whole server.

--
* Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD



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