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I have migrated a file based Xen guest to LVM using

dd bs=1M if=/dev/zero of=/dev/vg00/vm10
qemu-img convert ~/vm10.qcow2 -O raw /dev/vg00/vm10

and changed the Xen domain file for the VM to use the LV instead of the old file.

The VM boots up, and now on the Xen host would I like to make a snapshot of the running VM.

# lvcreate --size 10G --snapshot --name vm10-snapshot /dev/vg00/vm10 
  Logical volume "vm10-snapshot" created
# mount /dev/vg00/vm10-snapshot /mnt/snapshot/
mount: you must specify the filesystem type
# dmesg |tail
EXT3 FS on dm-3, internal journal
EXT3-fs: mounted filesystem with ordered data mode.
hfs: unable to find HFS+ superblock
VFS: Can't find ext3 filesystem on dev dm-4.
hfs: unable to find HFS+ superblock
hfs: unable to find HFS+ superblock
VFS: Can't find ext3 filesystem on dev dm-2.
hfs: unable to find HFS+ superblock
hfs: unable to find HFS+ superblock
hfs: unable to find HFS+ superblock

For some reason it can't see it is an EXT3 filesystem. I have also tried to mount with -t ext3, but still didn't mount.

# lvdisplay 
  --- Logical volume ---
  LV Name                /dev/vg00/vm10
  VG Name                vg00
  LV UUID                I1y1vQ-Bac5-5jwW-melh-TY5h-l9NO-qaelKk
  LV Write Access        read/write
  LV snapshot status     source of
                         /dev/vg00/vm10-snapshot [active]
  LV Status              available
  # open                 2
  LV Size                8.00 GB
  Current LE             2048
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:2

  --- Logical volume ---
  LV Name                /dev/vg00/vm10-snapshot
  VG Name                vg00
  LV UUID                GWsOx3-TPpr-GW64-uiMz-u1YN-QU4h-l0Kala
  LV Write Access        read/write
  LV snapshot status     active destination for /dev/vg00/vm10
  LV Status              available
  # open                 0
  LV Size                8.00 GB
  Current LE             2048
  COW-table size         10.00 GB
  COW-table LE           2560
  Allocated to snapshot  0.00% 
  Snapshot chunk size    4.00 KB
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:4

#

What could the problem be?

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3 Answers 3

up vote 2 down vote accepted

Unless you have an odd setup (i.e., please post your Xen config) /dev/vg00/vm10 is a disk image, not a filesystem. That is, it has a partition table and filesystems within it. Try doing a fdisk -l /dev/vg00/vm10 to see if this is the case. To mount an filesystem within that image you need to use a loopback mount. See, e.g., http://wiki.edseek.com/guide:mount_loopback

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Maybe you need to change the snapshot volume state to available? Try lvchange -a y /dev/vg00/vm10

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Good point, but it wasn't that. I have now updated the question with that the output from lvdisplay. –  Sandra Sep 20 '10 at 11:47

You need to define filesystem for you LV. use mkfs command to define the file system and then mention that file system while mounting the LV snapshot. it will work then

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