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[Linux-cluster] cmirror performance

  I've been trying out cmirror for a few months on a RHEL4U4 cluster and
it's now working very well for me, although I've noticed that it does
have a bit of a performance hit.

  My set-up has a 32G GFS filesystem on a mirrored LV shared via AoE
(with jumbo frame support). Just using dd with a 4k blocksize to write
files on the same LV when it's mirrored and then unmirrored shows a big
difference in speed:

    Unmirrored: 12440kB/s
    Mirrored:    2969kB/s

which I wasn't expecting as my understanding is that the cmirror design
introduces very little overhead.

  The two legs of the mirror are on separate, identical AoE servers and
the filesystem is mounted on 3 out of 6 nodes in the cluster. This is
with the cmirror-kernel_2_6_9_19 tagged version and I've tried with both
core and disk logs.

  I suspect a bad interaction between cmirror and something else, but
I'm not sure where to start looking. Any ideas?



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