How can I mount a root virtual disk from another VM on my CentOS ? In order to change some files on it, not to use it as a real root disk.

there are 2 partitions on my virtual disk : /boot (Linux partition) and / (LVM)

I've already added it to my VM by using virsh attach-disk, then I can see it on my CentOS but I can't use it :

mount /dev/vdx /mnt/test
mount: unknown filesystem type 'LVM2_member'

And something really weird, if I add my disk on my VM template, my centOS use the /boot partition of it (/dev/vdc1) instead of the /boot partition (/dev/vda1) that it has to use...

Thanks for help

[EDIT] :

This is the actual situation and my problem :

  • VM1 has virtual drive #1, provided by hypervisor (logical volume LVM). This VM is KO, I can't boot it because of I/O errors.
  • VM2 has virtual drive #2, provided by hypervisor (LVM too). It works fine.

The 2 virtual drives are cut like that on their VM : - /dev/vda1 = /boot (Linux partition) - /dev/vda2 = / (LVM partition)

Problem : I need to recover datas from VM1.

So I've already attach vd#1 to VM2. On VM2 I can see that drive, as /dev/vdc, but how can I mount it and read it ?


@psusi and @Halfgaar

Hypervisor :

virsh attach-disk my_VM /dev/vgdata/disk10 vdc

on my VM :

ls -la /dev/vd*

brw-rw---- 1 root disk 252, 0 Dec 22 11:22 /dev/vda
brw-rw---- 1 root disk 252, 1 Dec 22 11:22 /dev/vda1
brw-rw---- 1 root disk 252, 2 Dec 22 11:22 /dev/vda2
brw-rw---- 1 root disk 252, 16 Dec 22 11:22 /dev/vdb
brw-rw---- 1 root disk 252, 32 Dec 22 11:48 /dev/vdc
brw-rw---- 1 root disk 252, 33 Dec 22 11:48 /dev/vdc1
brw-rw---- 1 root disk 252, 34 Dec 22 11:48 /dev/vdc2

vgchange -ay ok. But how can I activate the volumes inside /dev/vdc if they are the same as my actual VM?

  • May be the dm_mod module is not loaded. Can you check whether dm_mod is getting listed in the output of lsmod ? – Sree Dec 21 '14 at 4:46
  • It looks to me as though you're connecting the wrong device. It kind of looks like you're attaching the physical volume of a LVM. Can you edit your post and specify your storage arrangement? Are all VM disks on a logical volume? Or are partitions (not disks) on LVs? – Halfgaar Dec 21 '14 at 8:38

In addition to my request for more information: Whether you have a virtual disk on a logical volume or a file, the following will work.

You can do fdisk -lu <diskimage> and see the partition table. Then you can see the offset at which your partition begins (except if it's GPT...). Then use losetup -d /dev/loop0 -o 512 * <startsector> <diskimage>. Then you can mount /dev/loop0 on any point. Make sure you mount with -o ro first, in case something went wrong. And, backup your image if you can.

If you do have a GPT partition table, I don't know from the top of my head how. You can probaly obtain the same info with gdisk of parted

If you have a Xen setup that serves partitions to the guest instead of disks, it's even easier; you can just mount it directly. But, I suspect you don't have that, even though you didn't say.

  • That won't work when the guest is using lvm – psusi Dec 21 '14 at 14:59
  • Are you sure? Why not? – Halfgaar Dec 21 '14 at 17:33
  • Because an lvm physical volume is not a filesystem you can mount; you have to mount the logical volume. – psusi Dec 21 '14 at 17:53
  • oh, you mean if the guest has LVM inside. Yeah, true... – Halfgaar Dec 21 '14 at 22:07

Your guest is using lvm so you need ot mount the logical volume, not the physical volume. Make sure vgchange -ay activates it and the logical volumes should appear in /dev/mapper.

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