33.7. Accessing data from a guest disk image

Procedure 33.1. Accessing guest image data

1. Install the kpartx package.

   # yum install kpartx
   

2. Use kpartx to list partition device mappings attached to a file-based storage image. This example uses a image file named guest1.img.

   # kpartx -l /var/lib/libvirt/images/guest1.img
   loop0p1 : 0 409600 /dev/loop0 63
   loop0p2 : 0 10064717 /dev/loop0 409663

   guest1 is a Linux guest. The first partition is the boot partition and the second partition is an EXT3 containing the root partition.
3. Add the partition mappings to the recognized devices in /dev/mapper/.

   # kpartx -a /var/lib/libvirt/images/guest1.img

   1. Test that the partition mapping worked. There should be new devices in the /dev/mapper/ directory

      # ls /dev/mapper/
      loop0p1
      loop0p2
      

      The mappings for the image are named in the format loopXpY.
4. Mount the loop device which to a directory. If required, create the directory. This example uses /mnt/guest1 for mounting the partition.

   # mkdir /mnt/guest1
   # mount /dev/mapper/loop0p1 /mnt/guest1 -o loop,ro

5. The files are now available for reading in the /mnt/guest1 directory. Read or copy the files.
6. Unmount the device so the guest image can be reused by the guest. If the device is mounted the guest cannot access the image and therefore cannot start.

   # umount /mnt/tmp

7. Disconnect the image file from the partition mappings.

   # kpartx -d /var/lib/libvirt/images/guest1.img

1. Add the partition mappings for the guest1.img to the recognized devices in the /dev/mapper/ directory.

   # kpartx -a /var/lib/libvirt/images/guest1.img

2. In this example the LVM volumes are on a second partition. The volumes require a rescan with the vgscan command to find the new volume groups.

   # vgscan
   Reading all physical volumes . This may take a while...
   Found volume group "VolGroup00" using metadata type lvm2

3. Activate the volume group on the partition (called VolGroup00 by default) with the vgchange -ay command.

   # vgchange -ay VolGroup00
   2 logical volumes in volume group VolGroup00 now active.

4. Use the lvs command to display information about the new volumes. The volume names (the LV column) are required to mount the volumes.

   # lvs
   LV VG Attr Lsize Origin Snap% Move Log Copy%
   LogVol00 VolGroup00 -wi-a- 5.06G
   LogVol01 VolGroup00 -wi-a- 800.00M

5. Mount /dev/VolGroup00/LogVol00 in the /mnt/guestboot/ directory.

   # mount /dev/VolGroup00/LogVol00 /mnt/guestboot

6. The files are now available for reading in the /mnt/guestboot directory. Read or copy the files.
7. Unmount the device so the guest image can be reused by the guest. If the device is mounted the guest cannot access the image and therefore cannot start.

   # umount /mnt/

8. Disconnect the volume group VolGroup00

   # vgchange -an VolGroup00

9. Disconnect the image file from the partition mappings.

   # kpartx -d /var/lib/libvirt/images/guest1.img