LVM snapshots can be really handy when you’re trying to take a backup of a running virtual machine. However, mounting the snapshot can be tricky if the logical volume is partitioned.
I have a virtual machine running zoneminder on one of my servers at home and I needed to take a backup of the instance with rdiff-backup. I made a snapshot of the logical volume and attempted to mount it:
[root@i7tiny ~]# lvcreate -s -n snap -L 5G /dev/vg_i7tiny/vm_zoneminder Logical volume "snap" created [root@i7tiny ~]# mount /dev/vg_i7tiny/snap /mnt/snap/ mount: wrong fs type, bad option, bad superblock on /dev/mapper/vg_i7tiny-snap, missing codepage or helper program, or other error In some cases useful info is found in syslog - try dmesg | tail or so
Oops. The logical volume has partitions. We will need to mount the volume with an offset so that we can get the right partition. Figuring out the offset can be done fairly easily with fdisk:
[root@i7tiny ~]# fdisk -l /dev/vg_i7tiny/vm_zoneminder Disk /dev/vg_i7tiny/vm_zoneminder: 53.7 GB, 53687091200 bytes 255 heads, 63 sectors/track, 6527 cylinders, total 104857600 sectors Units = sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disk identifier: 0x0007a1d5 Device Boot Start End Blocks Id System /dev/vg_i7tiny/vm_zoneminder1 * 2048 1026047 512000 83 Linux /dev/vg_i7tiny/vm_zoneminder2 1026048 102825983 50899968 83 Linux /dev/vg_i7tiny/vm_zoneminder3 102825984 104857599 1015808 82 Linux swap / Solaris
It looks like we have a small boot partition, a big root partition and a swap volume. We want to mount the second volume to copy files from the root filesystem. There are two critical pieces of information here that we need:
- the sector where the partition starts (the Start column from fdisk)
- the number of bytes per sector (512 in this case — see the third line of the fdisk output)
Let’s calculate how many bytes we need to skip when we mount the partition and then mount it:
[root@i7tiny ~]# echo "512 * 1026048" | bc 525336576 [root@i7tiny ~]# mount -o offset=525336576 /dev/mapper/vg_i7tiny-snap /mnt/snap/ [root@i7tiny ~]# ls /mnt/snap/ bin boot dev etc home lib lib64 lost+found media mnt opt proc root run sbin srv sys tmp usr var
The root filesystem from the virtual machine is now mounted and we can copy some files from it. Don’t forget to clean up when you’re finished:
[root@i7tiny ~]# umount /mnt/snap/ [root@i7tiny ~]# lvremove -f /dev/vg_i7tiny/snap Logical volume "snap" successfully removed
If you need to do this with file-backed virtual machine storage or with a flat file you made with dd/dd_rescue, read my post from 2010 about tackling that similar problem.