On Mon, Jun 18, 2012 at 11:33:50PM +0200, Spelic wrote: > Hello all > I am doing some testing of dm-thin on kernel 3.4.2 and latest lvm > from source (the rest is Ubuntu Precise 12.04). > There are a few problems with ext4 and (different ones with) xfs > > I am doing this: > dd if=/dev/zero of=zeroes bs=1M count=1000 conv=fsync > lvs > rm zeroes #optional > dd if=/dev/zero of=zeroes bs=1M count=1000 conv=fsync #again > lvs > rm zeroes #optional > ... > dd if=/dev/zero of=zeroes bs=1M count=1000 conv=fsync #again > lvs > rm zeroes > fstrim /mnt/mountpoint > lvs [snip ext4 problems] > On xfs there is a different problem. > Xfs apparently correctly re-uses the same blocks so that after the > first write at 140MB/sec, subsequent overwrites of the same file are > at full speed such as 350MB/sec (same speed as with non-thin lvm), > and also you don't see space occupation going up at every iteration > of dd, either with or without rm in-between the dd's. [ok actually > now retrying it needed 3 rewrites to stabilize allocation... > probably an AG count thing.] That's just a characteristic of the allocation algorithm. It's not something that you see in day-to-day operation of the filesystem, though, because you rarely remove and rewrite a file like this repeatedly. So in the real world, performance will be more like ext4 when you are running workloads where you actually store data for longer than a millisecond... Expect that the 140MB/s number is the normal performance case, because as soon as you take a snapshot, the overwrite requires new blocks to be allocated in dm-thinp. You don't get thinp for nothing - it has an associated performance cost as you are now finding out.... > However the problem with XFS is that discard doesn't appear to work. > Fstrim doesn't work, and neither does "mount -o discard ... + rm > zeroes" . There is apparently no way to drop the allocated blocks, > as seen from lvs. This is in contrast to what it is written here > http://xfs.org/index.php/FITRIM/discard which declare fstrim and > mount -o discard to be working. I don't see why it wouldnt be if the underlying device supports it. Have you looked at a block trace or an xfs event trace to see if discards are being issued by XFS? Are you getting messages like: XFS: (dev) discard failed for extent [0x123,4096], error -5 in dmesg, or is fstrim seeing errors returned from the trim ioctl? > Please note that since I am above MD raid5 (I believe this is the > reason), the passdown of discards does not work, as my dmesg says: > [160508.497879] device-mapper: thin: Discard unsupported by data > device (dm-1): Disabling discard passdown. > but AFAIU, unless there is a thinp bug, this should not affect the > unmapping of thin blocks by fstrimming xfs... and in fact ext4 is > able to do that. Does ext4 report that same error? Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
