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

[Mr. Teo En Ming (Zhang Enming)]

Thank you for your advice!

Is it called LVM snapshot?

-- 
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2009/11/12 Veli-Pekka Kestilä <fedora at guagua.fi>

> 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
>
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