I don't think COW in general is a big issue for databases. You can get pretty good performance for ZFS (very stable and consistent behavior), for example. The C

I don't think COW in general is a big issue for databases. You can get pretty good performance for ZFS (very stable and consistent behavior), for example. The COW is not free, of course, but you get interesting features in return, and if you need them (e.g. snapshots), it's usually much better than LVM + non-COW filesystem. The fact that some COW filesystem perform poorly does not mean all COW filesystems do.