On Tue, 30 Oct 2012, Tim Connors <tconnors@rather.puzzling.org> wrote:
if i had to guess, i'd say that there are probably some cases where LVM (with its nearly direct raw access to the underlying disks) would be faster than ZFS zvols but in most cases, ZFS' caching, compression, COW and so on would give the performance advantage to ZFS.
Just make sure the sum of the zvols remains below 80% of the total disk usage, I guess. ext4 + lvm can effectively use more than 99% of disk space (I've done it for years), but the moment you try to do lots of rewrites on a device on zfs, the lack of a free-space-cache that btrfs has means that the highly fragmented nature of the remaining 20% of space makes zfs completely unusable the first time you try it (multi-minute pauses and 250kB/s write rates vs 100MB/s. I quickly bought new disks).
That's one of several bad reports I've read about ZFS performance on mostly full filesystems. But it shouldn't be a big deal. I've got some servers that are all less than 25% full which I plan to convert to ZFS, even with snapshots I doubt that they will go over 50%. For my home server I currently have 1TB of RAID-1 which is more than 90% full (really bad given all the different Ext4 filesystems some of which are 99% full). When I convert that to ZFS on 3TB disks it will be a lot easier and I won't have issues of filesystem A filling while filesystem B is empty. If I upgrade my home server from 3TB disks when they have 2.5TB of data instead of waiting until they have 3TB of data it won't be that much of a big deal.
I'm actually regretting my move to zfs because of it - I can hardly afford to repeat the month it took to rsync the backuppc pool to zfs in the first place. If ext4+md aint broke, don't fix it. Sure it sucked, but the alternatives suck harder.
Ext4+md is broken. Linux software RAID has no support for recovering from silent data corruption, not even on RAID-6. Such corruption is becoming more common as data volumes steadily increase. We need BTRFS/ZFS features for data integrity. We also need something better than Ext4 fsck times. -- My Main Blog http://etbe.coker.com.au/ My Documents Blog http://doc.coker.com.au/