Date: Mon, 02 May 2005 15:22:58 +0200 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: Eric Anderson <anderson@centtech.com> Cc: freebsd-performance@freebsd.org Subject: Re: Very low disk performance on 5.x Message-ID: <17479.1115040178@critter.freebsd.dk> In-Reply-To: Your message of "Mon, 02 May 2005 08:16:12 CDT." <4276281C.6060209@centtech.com>
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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.
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