Re: [Xen-users] Using LVM snapshots to backup NTFS partitions forwindows guests

["Amos Shapira" <amos.shapira@xxxxxxxxx>]

On 24/10/2007, James Harper <james.harper@xxxxxxxxxxxxxxxx> wrote:

> Hi everyone!
>
> I've been reading this list for a while and I'm thinking that the best
> way to do backups is using LVM snapshots as discussed here several
> times.
>
> The problem is that when i use a LVM logical volume as windows disk,
> windows writes the partition table onto the LV so the LV itself is not
> a partition - it's a whole disk.
>
> lvm/device-mapper does not create the partition devices in /dev so i
> cannot mount/access the ntfs partitions with anything but fdisk, and i
> would like to use ntfsclone or similar.
>
> Is there any way around this?
>

In the past I have done this by using loopback with an offset, but you need to do a few sums to get that right (eg figure out where in the disk image each partition starts etc).

Not very complicated. I've been doing that a few times in the last few days while trying to get CentOS 5 installer running under Debian Etch with Xen 3.0.3.

"fdisk -l -u /dev/xen/lvm-name" will give you the offsets. The "Units = " line will give you the multiplier (always was 512 for me).
Then you can do:

Find an available loop device:

# losetup -f
/dev/loop1

Setup the loop device with the right offset, the `expr...` backtick will multiply the number you got from fdisk by 512:

# losetup -o `expr offset-from-fdisk \* 512` /dev/loop1 image-file-or-lvm

After that yuou can do
# mount /dev/loop1 /mnt/mount-point

This is on Debian Etch (mount package version 2.12).

Hope this helps,

--Amos

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