[Lustre-discuss] MDT hardlink snapshots?

[Daire Byrne]

Hi,

I was wondering if it is possible to mount an MDT device with ldiskfs, hardlink a dir tree and then remount (lustre) without Lustre clients getting confused? Are hardlinks in Lustre a "bad idea" for any reason (i.e. linking files into a different dir tree). I was toying with the idea (madcap Fridays) of doing hardlink style file based snapshots by doing something like this:

  [root at mds ~] umount -f /dev/lustre/mdt1
  [root at mds ~] mount -t ldiskfs /dev/lustre/mdt1 /mnt/lustre/mdt1
  [root at mds ~] cp -fal /mnt/lustre/mdt1/ROOT/blah /mnt/lustre/mdt1/ROOT/blah0
  [root at mds ~] mount -t lustre /dev/lustre/mdt1 /mnt/lustre/mdt1

Something akin to "rsync -H --link-dest" backups (dirvish). A quick benchmark suggests that you can link around 100 million files in 6 hours. It is debatable how viable it would be to take a filesystem down for that long - maybe a a weekend operation. I was just curious if there was anything in Lustre that makes this impossible/bad. This would probably be impossible in Lustre 2.0 (CMD and changelogs).

Hard linking over the network is obviously an order of magnitude slower although I have not tested yet with lots of clients. Eagerly awaiting unlimited LVM shared exception store snapshots....

  http://people.redhat.com/mpatocka/patches/kernel/new-snapshots/devel/

Daire