Re: Very low disk performance on 5.x

From: Eric Anderson (anderson_at_centtech.com)
Date: 05/02/05

  • Next message: Steven Hartland: "Re: Very low disk performance on 5.x" 
    Date: Mon, 02 May 2005 08:27:52 -0500
To: Poul-Henning Kamp <phk@phk.freebsd.dk>

    Poul-Henning Kamp wrote:
    > 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.

    If I write a 10GB file to disk (RAID array has 1GB cache, system has 1GB
    memory), then I should definitely see better read performance reading
    that same file back to /dev/null than writing it, right?

    How about this - you tell me what test to run, and I'll do it (as long
    as it doesn't destory my data).

    Eric 

    -- 
------------------------------------------------------------------------
Eric Anderson        Sr. Systems Administrator        Centaur Technology
A lost ounce of gold may be found, a lost moment of time never.
------------------------------------------------------------------------
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  • Next message: Steven Hartland: "Re: Very low disk performance on 5.x"