Comment 7 for bug 113713

For the record, my entire system is LVM-on-MD-RAID1. While I some times seem some level of slow-down, it's never to the degree you're seeing. However, I don't have long-running snapshots. They're always temporary, and they're of unmounted filesystems. Snapshots are, by nature, pretty slow. When you create them, any deltas between the origin and the snapshot need to be stored in the COW file for the snapshot, so doing operations that make lots of changes to the origin partition (like, say "find" which touches the atime of ALL the inodes in the find) can send your COW file spinning for a while. There is a lot of IO done to manage a long-term snapshot, which is why they're not really recommended. They're good for snapshotting for backups, and then releasing them, though.

Check your IO levels with "iostat 5" (and ignore the first report, as that's just a running average). That may show the disk activity. You can map dm-* to names via /dev/mapper entries and their minor number. i.e. dm-19 is 254, 19 in /dev/mapper.

Did prior releases of the kernel behave better?