Re: Very low disk performance on 5.x

From: Robert Watson (rwatson_at_FreeBSD.org)
Date: 05/02/05

  • Next message: Sten Spans: "Re: Very low disk performance on 5.x" 
    Date: Mon, 2 May 2005 21:48:19 +0100 (BST)
To: Poul-Henning Kamp <phk@phk.freebsd.dk>

    On Mon, 2 May 2005, Poul-Henning Kamp wrote:

    >> I'm quite willing to test and optimise things but so far no one has
    >> had any concrete suggestions on that to try.
    >
    > First thing I heard about this was a few hours ago. (Admittedly, my
    > email has been in a sucky state last week, so that is probably my own
    > fault).

    This is because I resent a message on the -performance list to you and
    Scott Long, and Soren because I was concerned that the thread was
    identifying a reproduceable problem for several people, but no one had
    really bit in to try and diagnose it :-).

    It seems likely we're hurting at a couple of levels, and it would be good
    to identify what's appearing at what level. I think comparing dd results
    on the total disk device directly as a starting point is a good place to
    begin. I'm not sure if we've seen Linux and FreeBSD dmsg output yet, but
    if nothing else it would be good to confirm if the drivers on both systems
    negotiate the same level of throughput to each drive.

    The next thing that would be quite nice to measure is the rate of I/O
    transactions per second we can get to the disk using the disk device
    directly, with a minimal transaction size. I have a vague recollection
    that you have to be careful in Linux because their character device nodes
    for disk devices are buffered, and you really want unbuffered I/O.

    After transactions/sec, broad sweeps across well-understood areas of the
    disk would be a good thing to do, again using the device node directly.

    Isolating the impact of the file system will be important...

    Robert N M Watson
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  • Next message: Sten Spans: "Re: Very low disk performance on 5.x"

    Relevant Pages

    • Re: Very low disk performance on 5.x
      ... >> transactions per second we can get to the disk using the disk device ... sequentially reading from my 4-disk RAID5 ) via the filesystem. ... $ sysctl vfs.read_max=16 ...
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    • Re: Very low disk performance on 5.x
      ... > The next thing that would be quite nice to measure is the rate of I/O ... > transactions per second we can get to the disk using the disk device ... dramatic effect on sequential read performance. ...
      (freebsd-performance)