Re: Apparent strange disk behaviour in 6.0

From: Brian Candler (B.Candler_at_pobox.com)
Date: 07/30/05

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    Date: Sat, 30 Jul 2005 18:15:36 +0100
To: Julian Elischer <julian@elischer.org>

    On Sat, Jul 30, 2005 at 03:29:27AM -0700, Julian Elischer wrote:
    > >Please use gstat and look at the service times instead of the
    > >busy percentage.
    > >
    > >
    >
    > The snapshot below is typical when doing tar from one drive to another..
    > (tar c -C /disk1 f- .|tar x -C /disk2 -f - )
    >
    > dT: 1.052 flag_I 1000000us sizeof 240 i -1
    > L(q) ops/s r/s kBps ms/r w/s kBps ms/w d/s kBps ms/d
    > %busy Name
    > 0 405 405 1057 0.2 0 0 0.0 0 0 0.0
    > 9.8| ad0
    > 0 405 405 1057 0.3 0 0 0.0 0 0 0.0
    > 11.0| ad0s2
    > 0 866 3 46 0.4 863 8459 0.7 0 0 0.0
    > 63.8| da0
    > 25 866 3 46 0.5 863 8459 0.8 0 0 0.0
    > 66.1| da0s1
    > 0 405 405 1057 0.3 0 0 0.0 0 0 0.0
    > 12.1| ad0s2f
    > 195 866 3 46 0.5 863 8459 0.8 0 0 0.0
    > 68.1| da0s1d
    >
    > even though the process should be disk limitted neither of the disks is
    > anywhere
    > near 100%.

    Are ad0 and da0 both arrays?

    One IDE disk doing 405 reads per second (2.5ms per seek) is pretty good. A
    7200rpm drive would have a theoretical average seek time of 1/(7200/60)/2 =
    4.2ms, or 7200/60*2 = 240 ops per second. It can be better with read-ahead
    caching.

    But if really is only 12.1% busy (which the 0.3 ms/r implies), that means it
    would be capable of ~3350 operations per second... that's either a seriously
    good drive array with tons of cache, or the stats are borked :-)

    With a single bog-standard IDE drive tarring up a directory containing some
    large .iso images, and piping the output to /dev/null, I get:

     L(q) ops/s r/s kBps ms/r w/s kBps ms/w %busy Name
        1 389 388 49318 2.4 1 24 1.4 90.6| ad0s3d

    And tarring up /usr/src (again piping to /dev/null) I get:

        1 564 564 5034 1.7 0 0 0.0 95.7| ad0s2e

    This is with 5-STABLE as of 2005-05-13 (i.e. a bit after 5.4-RELEASE), and
    an AMD 2500+ processor. Interestingly, I get a much higher kBps than your
    ad0 - although I'm not actually writing the data out again.

    Maybe it would be interesting to pipe the output of your tar to /dev/null
    and see how the read performance from ad0 compares with your measured <read
    from ad0 plus write to da0> performance? Then try just reading from da0?

    Regards,

    Brian.
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