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From: Gregory Stark <stark@enterprisedb.com>
To: Matthew <matthew@flymine.org>
Cc: pgsql-performance@postgresql.org
Subject: Re: RAID arrays and performance
Date: Tue, 04 Dec 2007 12:40:32 +0000
Message-ID: <87lk8aiqf3.fsf@oxford.xeocode.com> (raw)
In-Reply-To: <Pine.LNX.4.58.0712041213490.3731@aragorn.flymine.org>
References: <Pine.LNX.4.58.0712041213490.3731@aragorn.flymine.org>

"Matthew" <matthew@flymine.org> writes:

> Does Postgres issue requests to each random access in turn, waiting for
> each one to complete before issuing the next request (in which case the
> performance will not exceed that of a single disc), or does it use some
> clever asynchronous access method to send a queue of random access
> requests to the OS that can be distributed among the available discs?

Sorry, it does the former, at least currently.

That said, this doesn't really come up nearly as often as you might think.
Normally queries fit mostly in either the large batch query domain or the
small quick oltp query domain. For the former Postgres tries quite hard to do
sequential i/o which the OS will do readahead for and you'll get good
performance. For the latter you're normally running many simultaneous such
queries and the raid array helps quite a bit.

-- 
  Gregory Stark
  EnterpriseDB          http://www.enterprisedb.com
  Ask me about EnterpriseDB's On-Demand Production Tuning



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