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    SubjectRe: XFS write speed drop

    >> CASE 1: Copying from one disk to another
    >> ========================================
    >> Copying a compiled 2.6.17-rc4 tree; 306907 KB in 28566 files in 2090
    >> directories.
    >
    >OK, we can call this a metadata intensive workload - lots of small
    >files, lots of creates. Barriers will hurt the most here, as we'd
    >already have been log I/O bound most likely, and I'd expect barriers
    >to only slow that further.
    >
    Yes and the most important thing is that someone made -o barrier the
    default and did not notice. Someone else? :-D

    >Yep, note the user/sys shows no change, we're basically IO bound in
    >both tests, and barriers are hurting (as expected).


    >> CASE 2: Removing
    >> ================
    >> Remove the copies we created in case 1.
    >>
    >> 15:45 (none):/tmp # mount /dev/hdc2 /D -o barrier
    >> 15:45 (none):/D # time rm -Rf kernel-0
    >> real 3m31.901s
    >> user 0m0.050s
    >> sys 0m3.140s
    >
    >versus:
    >
    >> 15:49 (none):/ # mount /dev/hdc2 /D -o nobarrier
    >> 15:49 (none):/D # time rm -Rf kernel-1
    >>
    >> real 0m53.471s
    >> user 0m0.070s
    >> sys 0m1.990s
    >> 15:50 (none):/D # cd /
    >> 15:50 (none):/ # umount /D
    >
    >Also metadata intensive, of course. All the same issues as above,
    >and the same techniques should be used to address it.
    >
    >Odd that the system time jumped here. Roughly the same decrease in
    >performance though (3-4x).

    You may, or may not, take that serious. I only ran each test once (since
    3m31s vs 53s shows "enough" of the issue).

    >So, I agree, you're seeing the cost of write barriers here, but I
    >don't see anything unexpected (unfortunately for you, I guess).
    >
    >The FAQ entry above will explain why they're enabled by default.
    >Try the v2 log change I suggested, hopefully that will mitigate
    >the problem somewhat. Alternatively, buy some decent hardware if
    >you're after performance. ;)
    >
    Well as mentioned, -o nobarrier solved it, and that's it. I do not actually
    need barriers (or an UPS, to poke on another thread), because power
    failures are rather rare in Germany.


    Jan Engelhardt
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