[linux-lvm] mounting snapshot errors

Chris Beck cbeck at gene.concordia.ca
Wed Jan 28 13:23:07 UTC 2004

Yah, I saw some stuff about this in the LVM-HOWTO:


| If the snapshot is of an XFS filesystem, the xfs_freeze command should 
be used to quiesce the filesystem before creating the snapshot. (if the 
filesystem is mounted) |

|# xfs_freeze -f /mnt/point; lvcreate -L592M -s -n dbbackup /dev/ops/databases; xfs_freeze -u /mnt/point 

If you are using XFS as the filesystem you will need to add the nouuid option to the mount command:

# mount /dev/ops/dbbackup /mnt/ops/dbbackup -onouuid,ro
Previously, the norecovery option was suggested to allow the mounting of XFS snapshots.
It has been recommended not to use this option, but to instead use xfs_freeze to quiesce the filesystem before creating the snapshot.



It is whispered that Greg Freemyer was heard, on or about 1/28/2004 
12:34 PM to say:

>On Wed, 2004-01-28 at 02:28, R Dicaire wrote:
>>On Tue, 2004-01-27 at 21:53, Little, Chris wrote:
>>>is the filesystem you are snapshotting still mounted when you execute the
>>>snapshot?  They need to be offline before the snapshot takes place.
>>Yes, they are still mounted when I take a snapshot, I must have missed that
>>in the docs, that in itself poses a problem as I then cant umount the volumes
>>without killing services. I'd understood the snapshotting process as being
>>able to snapshot a live filesystem, and then mount the snapshot to perform
>>backups from it instead of the live filesystem. Whats the purpose of the
>>--size arg to lvcreate when taking a snapshot, I thought it was a buffer
>>to hold data that'd normally be written to disk but cant when the
>>snapshot exists? I'm confused.
>I snapshot live mounted volumes every day.  (LVM 1.x, kernel 2.4.21)
>It works.
>I use xfs.
>For LVM snapshot to work with mounted filesystems you either have to
>have the vfs-lock patch applied or invoke a filesystem freeze from user
>space prior to creating the snapshot.
>With xfs, I use the xfs_freeze command from userspace.
>Then on the xfs mount, I have to use the "-o ro,nouuid" flag to mount
>the snapshot because the master volume and the  snapshot have the same

Chris Beck / Y.A.B.A. / Fungal Genomics
CFSG / Concordia University
"La loi dans sa majestueuse égalité, interdit à tous, aux riches comme
aux pauvres de dormir sous les ponts, de coucher dans la rue et de voler
du pain." -- Anatole France (Les Lys Rouge - 1894)

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