[CentOS] LVM performance with snapshots

Wed Mar 14 00:41:16 UTC 2007
t m <bigendian at gmail.com>

I've been experimenting with LVM and snapshots under Centos 4.4
recently, and have found performance to be horrible while a snapshot
of a logical volume exists.

I expected some performance degradation with a snapshot in place due
to my understanding of the copy-on-write behavior, but my system
becomes almost unusable when writing to my system with a snapshot in
place.  For example, I can write a 4 gigabyte file in about 16 seconds
with no snapshots in place, but the same file either takes about 4
minutes to write, or my systems seems  to hang when a snapshot exists.
 The top and iostat commands seem to show very high iowait times and
very low actual I/O throuhput, and pdflush seems to be quite busy
doing something.  Removing the snapshot immediately brought
performance back to normal.

Are LVM snapshots not recommended with Centos, or are there
recommended ways to tune it for better performance?

Thanks for any input!