Mounting xen disk images. February 7, 2011

I run a Xen vm system which in turn hosts all of my linux servers. Recently, I had reason to mount the disk image from one of the vm’s on the xen host.

I’ve found little documentation on this, so I thought i’d post what I ended up doing.

First, make sure the vm is down. Then you can mount it’s disk image.

I used kpartx to mount the image as a loop.

[root@chaos ~]# kpartx -av /var/lib/xen/images/interchange_os.img
add map loop0p1 : 0 401562 linear /dev/loop0 63
add map loop0p2 : 0 2104515 linear /dev/loop0 401625
add map loop0p3 : 0 18458685 linear /dev/loop0 2506140

The VM uses LVM for its disk configuration, so I needed to them work some disk magic to get it to load the PV, VG, and finally LV’s.

First, scan for new PV’s.

[root@chaos ~]# pvscan /dev/mapper/loop0p3
  PV /dev/mapper/loop0p3   VG vg_root      lvm2 [8.78 GB / 0    free]
  PV /dev/sda3             VG VolGroup00   lvm2 [269.16 GB / 0    free]
  Total: 2 [277.94 GB] / in use: 2 [277.94 GB] / in no VG: 0 [0   ]

Then scan for new VG’s.

[root@chaos ~]# vgscan
  Reading all physical volumes.  This may take a while…
  Found volume group “vg_root” using metadata type lvm2
  Found volume group “VolGroup00” using metadata type lvm2

Now, scan for new LV’s.

[root@chaos ~]# lvscan
  inactive          ‘/dev/vg_root/lv_slash’ [3.00 GB] inherit
  inactive          ‘/dev/vg_root/lv_home’ [1.00 GB] inherit
  inactive          ‘/dev/vg_root/lv_tmp’ [512.00 MB] inherit
  inactive          ‘/dev/vg_root/lv_var’ [3.28 GB] inherit
  inactive          ‘/dev/vg_root/lv_var_run’ [1.00 GB] inherit
  ACTIVE            ‘/dev/VolGroup00/lv_root’ [8.00 GB] inherit
  ACTIVE            ‘/dev/VolGroup00/lv_var’ [256.16 GB] inherit
  ACTIVE            ‘/dev/VolGroup00/lv_tmp’ [1.00 GB] inherit
  ACTIVE            ‘/dev/VolGroup00/lv_home’ [4.00 GB] inherit

And now, activate the VG’s

[root@chaos ~]# vgchange -ay
  5 logical volume(s) in volume group “vg_root” now active
  4 logical volume(s) in volume group “VolGroup00” now active

Now, you see that /dev/mapper contains my vg’s, and their lv’s.

[root@chaos ~]# ls /dev/mapper/
control  loop0p2  vg_root-lv_home   vg_root-lv_tmp  vg_root-lv_var_run  VolGroup00-lv_root  VolGroup00-lv_var
loop0p1  loop0p3  vg_root-lv_slash  vg_root-lv_var  VolGroup00-lv_home  VolGroup00-lv_tmp

All that’s neft to do now is mount them like any other local disk.

[root@chaos ~]# mount /dev/mapper/vg_root-lv_slash /mnt/
[root@chaos ~]# ls /mnt
bin  boot  dev  etc  home  lib  lost+found  media  misc  mnt  opt  proc  root  sbin  selinux  srv  sys  tmp  usr  var
[root@chaos ~]#

To dismount, simply unmount the volume, disable the VG, and then use kpartx to remove the loop.

[root@chaos ~]# umount /mnt/
[root@chaos ~]# vgchange -an vg_root
  0 logical volume(s) in volume group “vg_root” now active
[root@chaos ~]# kpartx -d /dev/loop0

Now, your image should be clean for your vm once more.