Date: Wed, 8 Sep 2010 22:13:05 +0200
From: Enrico Weigelt <weigelt@metux.de>
To: pgsql-admin@postgresql.org
Subject: Re: [NOVICE] - SAN/NAS/DAS - Need advises
Message-ID: <20100908201305.GA31637@nibiru.local>
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* Scott Marlowe <scott.marlowe@gmail.com> wrote:

> What really matters here is if the 8gb SAN is as fast as or faster
> than the DAS setup.  For most people measuring the speed of the
> interface is a lot like the famous Tom Lane quote about benchmarking
> jet fighters versus airliners by measuring the amount of runway they
> need.

If you really want to know what's better, you'll have to test the
candidates with real loads: record your real runtime load w/ blktrace
and replay them on the candidates.

Nominal performance parameters as insufficient as a dd or iobench test.

For example, take an IBM XIV or DS8k w/ thin provisioning: simply
writing zeros (or just the same pattern to all blocks) will give you
almost the raw bus speed (maybe adding a little bit overhead inside
the storage system), since spindles will idle. Caches (bein RAM or
SSD) can make the storage look way faster than it really is (in rare
cases, bad cache decisions could make also make it slower).

Synthetic tests don't allow you to predict how spindle seeks, bus
jams, etc will behave in real production. It *heavily* depends on
your workload which storage type (and concrete product) will perform
in production, nominal parameters are too vague for enterprise usage.
(no, you can't even tell that DAS is faster than SAN).

You need to test your workload and then decide which product to use.


cu
-- 
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 Enrico Weigelt, metux IT service -- http://www.metux.de/

 phone:  +49 36207 519931  email: weigelt@metux.de
 mobile: +49 151 27565287  icq:   210169427         skype: nekrad666
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