Re: Very low disk performance on 5.x

From: Poul-Henning Kamp (phk_at_phk.freebsd.dk)
Date: 05/02/05

  • Next message: Petri Helenius: "Re: Very low disk performance on 5.x" 
    To: Eric Anderson <anderson@centtech.com>
Date: Mon, 02 May 2005 15:22:58 +0200

    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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  • Next message: Petri Helenius: "Re: Very low disk performance on 5.x"

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