[linux-lvm] write performance with active snapshot

[Peter Daum <gator_ml@yahoo.de>]

Hi,

for an application I am just working on it looks like lvm snapshots would
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?

Regards,
                          Peter Daum