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[linux-lvm] Re: write performance with active snapshot


Larry Dickson wrote:
My guess is that you are getting the typical seek overhead. Have you tried making a volume group out of two separate RAID arrays (driving different spindles), and using lvdisplay --maps to make sure the parent volume is on one array, the snapshot(s) on the other?

That was my suspicion, too (although I could not imagine such an extreme
impact). Just for testing I added a single disk to the same volume group
and put the snapshot onto that disk - amazingly it made hardly any
difference (Actually, I'm almost glad about that, because the combination
of a 12-disk-array with a single disk would be under almost all other
aspects foolish).

One thing that does improve the performance a little (actually by 100%,
which in this case meens still pretty lousy 16 MB/sec) is to increase
the chunk size to 512kb. (I don't know yet, how this might affect
performance when dealing with many small files) ...


On 11/9/08, *Peter Daum* <gator_ml yahoo de <mailto:gator_ml yahoo de>> wrote:


    for an application I am just working on it looks like lvm snapshots
    be just what I need as far as functionality is concerned. Unfortunately,
    I am experiencing such a massive degradation in performance, that the
    result is almost useless.

    I'm working on a fairly fast machine (Quadcore, 8GB RAM) with a big
    hardware RAID array and lvm2 (Debian Lenny; Linux 2.6.26-1-amd64;
    LVM version:2.02.39 (2008-06-27)
    Library version: 1.02.27 (2008-06-25)
    Driver version:  4.13.0)

    Sequentially writing to a file (ext3) on a logical volume, I get a
    sustained performance of ~ 250 MB/sec. When I create a snapshot
    volume, the write throughput drops to 7-8 MB/secs (on the original
    volume; writing to the snapshot I see a significant degradation,
    but not nearly, as bad; read performance is o.k.).Is this "normal"
    or is there anything I can do to about it?

    I looked in this list and searched the WWW but couldn't find any
    concrete information on the performance impact of snapshots
    (except http://www.nikhef.nl/~dennisvd/lvmcrap.html).
    It seems like write performance should probably be less then 1/3
    of the original throughput, because every write to the source
    volume causes 3 I/O operations plus some overhead for meta data.
    More difficult to estimate would be the time lost by additional
    head movements. Still, a throughput degradation by a factor of 30
    seems pretty extreme.

    Any ideas?

                            Peter Daum

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