Re: Very low disk performance on 5.x

From: Arne (arne_woerner_at_yahoo.com)
Date: 05/01/05

  • Next message: Mike Tancsa: "64bit CPUs" 
    Date: Sun, 1 May 2005 04:23:51 -0700 (PDT)
To: Robert Watson <rwatson@FreeBSD.org>

    --- Robert Watson <rwatson@FreeBSD.org> wrote:
    > On Sat, 30 Apr 2005, Arne WXrner wrote:
    > > 3. The man page geom(4) of R5.3 says "The GEOM framework
    > > provides an infrastructure in which "classes" can per-
    > > form transformations on disk I/O requests on their path
    > > from the upper kernel to the device drivers and back.
    > >
    > > Could it be, that geom slows something down (in some boxes the
    > > reading
    > > ops are very slow; in my box the writing ops are very slow)?
    >
    > [...]
    > against a further offset region of ad0. If they're against the
    > same bits of disk, the main difference here will be the
    > additional processing of the layers in the stack. A little
    > bit of math is required to figure out the offset, but dd
    > should be usable to figure out the incremental cost.
    >
    I used the following commands:
    1. dd if=/dev/ad0s2a bs=16k count=10000 of=/dev/null
    2. dd if=/dev/ad0 iseek=2100357 bs=16k count=10000 of=/dev/null
    (I think I did the math correctly; indeed the read speed of my ad0
    varies between 45MB/sec (iseek=0) and 25MB/sec (in the end) with
    bs=128k).
    The results were nearly the same (both between 26MB/sec and
    28MB/sec). Maybe I should have done it in single user mode.

    My other hard disc ad1 (it is newer and bigger and faster and more
    furious) behaves better (but I can just try writing via the file
    system (ufs+s) and a final sync): The sync just needs .24sec of
    18.28 seconds; the read and write speed is nearly the same (about
    37MB/sec+/-1MB/sec).

    Is there anything else, that could help to find the reason for the
    difference between ad0's speed in R4.11 and R5.3?

    -Arne

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