Archive for November 16th, 2009


LVM Snapshots and XFS

Small note for everyone planing to use Linux LVM snapshots and using XFS at the same time. XFS has UUIDs which are unique identifiers of the filesystem. Two file systems with same UUID can not be mounted on the same server. Now, if we know that a snapshot of a logical volume represents a point-in-time copy of the original logical volume it doesn't take much time to realize that the filesystem on the snapshot is also a copy, thus it will have the same UUID as the filesystem on the original logical volume. So here is what happens when you try to mount the snapshot:

[root@server ~]# df -hT /var/lib/mysql_backup/
Filesystem    Type    Size  Used Avail Use% Mounted on
              xfs    25G   19G  6.3G  76% /var/lib/mysql_backup
[root@server ~]# lvcreate -s -n bkp-snap -L1G /dev/sys/mysql_backup
  Logical volume "bkp-snap" created
[root@server ~]# mount /dev/sys/bkp-snap /mnt/misc/
mount: wrong fs type, bad option, bad superblock on /dev/sys/bkp-snap,
       missing codepage or other
       In some cases useful info is found in syslog - try
       dmesg | tail or so
[root@server ~]# dmesg | tail -1
XFS: Filesystem dm-9 has duplicate UUID - can't mount

It doesn't look very good. There are two solution for this problem. One is to use nouuid option for mount command.

[root@server ~]# mount -o nouuid /dev/sys/bkp-snap /mnt/misc/
[root@server ~]# dmesg | tail -3
XFS mounting filesystem dm-9
Starting XFS recovery on filesystem: dm-9 (logdev: internal)
Ending XFS recovery on filesystem: dm-9 (logdev: internal)

Another option would be to change UUID of the filesystem on the snapshot using xfs_admin command.

[root@server ~]# xfs_admin -U generate /dev/sys/bkp-snap
Clearing log and setting UUID
writing all SBs
new UUID = 1bdcf6e1-62fb-47f2-83e4-dc398bb7a1cd
[root@server ~]# dmesg | tail -2
XFS mounting filesystem dm-9
Ending clean XFS mount for filesystem: dm-9

I am in favour of the first option (mount -o nouuid) since it does not perform any modification on the filesystem. It just feels safer, that's all... :)