From owner-freebsd-performance@FreeBSD.ORG  Mon May  2 13:23:01 2005
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To: Eric Anderson <anderson@centtech.com>
From: "Poul-Henning Kamp" <phk@phk.freebsd.dk>
In-Reply-To: Your message of "Mon, 02 May 2005 08:16:12 CDT."
             <4276281C.6060209@centtech.com> 
Date: Mon, 02 May 2005 15:22:58 +0200
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cc: freebsd-performance@freebsd.org
Subject: Re: Very low disk performance on 5.x 
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In message <4276281C.6060209@centtech.com>, Eric Anderson writes:
>Poul-Henning Kamp wrote:
>> In message <427626DC.5030702@centtech.com>, Eric Anderson writes:
>> 
>> 
>>>Don't mean to be terse here, but I'm talking about the same test done an 
>>>two different RAID5 configurations, with different disks, and not just 
>>>me - other users in this very thread see the same issue..
>> 
>> 
>> Uhm, if you are using RAID5 and your requests are not aligned and
>> sized after the RAID5 you should *expect* read performance to be poor.
>> 
>> If you your request ends up accessing two different blocks even just
>> once per stripe, this totally kills performance.
>
>Wouldn't this be a problem for writes then too?

I presume you would only compare read to write performance on a RAID5
device which has battery backed cache.

Without a battery backed cache (or pretending to have one) RAID5
write performance is abysmall no matter which alignment you use.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.