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From: Michael Stone <mstone+postgres@mathom.us>
To: pgsql-performance@postgresql.org
Subject: Re: Advice on RAID card
Date: Sun, 25 Sep 2005 14:13:52 -0400
Message-ID: <20050925181352.GA2241@mathom.us> (raw)
In-Reply-To: <873bntjb9p.fsf@stark.xeocode.com>
References: <IGEGINLEEDLANNHAAKCMOECJCCAA.gaulakh@ecmarket.com>
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	<43367937.40809@canaan.co.il>
	<ABE6B997-2C1E-43B1-9F18-28F54B1620FE@fastcrypt.com>
	<4336C57B.6090800@commandprompt.com>
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	<4336CEAA.7070208@commandprompt.com>
	<op.sxn3v7a1th1vuj@localhost>
	<873bntjb9p.fsf@stark.xeocode.com>

On Sun, Sep 25, 2005 at 01:41:06PM -0400, Greg Stark wrote:
>Also, Raid 5 is particularly inappropriate for write-heavy Database traffic.
>Raid 5 actually hurts write latency dramatically and Databases are very
>sensitive to latency.

Software raid 5 actually may have an advantage here. The main cause for
high raid5 write latency is the necessity of having blocks from each
disk available to calculate the parity. The chances of a pc with several
gigs of ram having all the blocks cached (thus not requiring any reads)
are higher than on a hardware raid with several hundred megs of ram. 

Mike Stone



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