[Fedora-xen] How to Backup and Restore MBR within Logical Volumes?

[Veli-Pekka Kestilä]

Mr. Teo En Ming (Zhang Enming) wrote:
> Hi All,
>
> I have several Xen virtual machines within logical volumes using LVM2. 
> I did not use disk images for performance reasons.
>
> # losetup /dev/loop1 /dev/virtualmachines/windows7-x64
> windows7-x64 is the logical volume.
> # kpartx -av /dev/loop1
>
> Then I would see the partitions of the virtual machine within a 
> logical volume, like so:
>
> /dev/mapper/loop1p1
> /dev/mapper/loop1p2
> /dev/mapper/loop1p3
>
> Now that I can access the partitions of the virtual machine within a 
> logical volume, I can use partimage or fsarchiver to backup the 
> partitions (provided the filesystem is supported by the archiver).
>
> But the problem is that I can only backup/clone the filesystems of my 
> virtual machine within a logical volume. I can't backup the Master 
> Boot Record (MBR) of the virtual machine within a logical volume. For 
> example,
>
> dd if=/dev/hda of=mbr.hda bs=512 count=1
>
> Because /dev/hda resides in a logical volume. The logical volume is a 
> virtual harddisk for my virtual machine.
You can do dd if=/dev/dev/loop1 of=mbr.dha bs=512 count=1 to make the 
copy of the boot block.
>
> I would like to know how to backup and also restore the MBR of my 
> virtual machine/guest operating system/domU within a logical volume 
> because the losetup and kpartx procedure only allows me access to the 
> partitions, not the MBR. A complete backup of a virtual machine (and 
> also a bare metal machine) includes the MBR and all filesystems.
>
> If there is a catastrophic failure with my logical volumes containing 
> domUs, I would like to
>
> 1) re-create the physical volume (PV)
>
> 2) re-create the volume group
>
> 3) assign the PV to the volume group
>
> 4) restore the LVM metadata, i.e. the configuration files for all the 
> logical volumes
>
> 5) restore the MBR of my domU
>
> 6) restore the filesystems of my domU
>
Of these steps you can recreate 1,2,3 and 4 just by keeping records of 
their parameters and using normal fdisk, pvcreate, vgcreate and lvcreate 
commands.

For the things inside of lvm you can use losetup to make the loopback 
device out of the lv where you want to install the quest operating 
system and then recreate mbr from the file. Problem probably is how to 
recreate the filesystems if your backup software cannot do it. Also with 
windows protected system files can be a problem if backup software 
doesn't support them.

One thing you could of course do is to make the system restore backup 
inside of the DomU when it's running. And then boot new DomU when 
starting up and use the system backup to restore. (of course this is 
still more work that pure dd from one place to another.)

Problem is that everything else than using dd can have it fair share of 
problems. If the domU can't be offline too long, you could allocate more 
space and use the lvm:s instant cloning features (can't remember the 
name just now. ) and then use dd with gzip or bzip to make the backup 
from this clone to keep the space requirements minimum for the stored 
copies. And have minimum disruption for the DomU.

-vpk