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!