How to Backup and Restore MBR within Logical Volumes?

Thu Nov 12 12:32:22 UTC 2009

On Thu, Nov 12, 2009 at 10:55 AM, Mr. Teo En Ming (Zhang Enming) <
space.time.universe at gmail.com> wrote:

>
>
> On Thu, Nov 12, 2009 at 10:45 AM, Bryn M. Reeves <bmr at redhat.com> wrote:
>
>> On Thu, 2009-11-12 at 09:23 +0000, 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.
>> >
>> > Conventionally, if I want to clone my virtual machines, I have to dd
>> > the LV to an image file. But this consumes a lot of time and harddisk
>> > space.
>> >
>> > So, instead of doing that, I want to use losetup and kpartx with my
>> > logical volumes, which contain operating systems of virtual machines.
>> >
>> > I can backup the filesystems of a virtual machine in this way:
>> >
>> > # losetup /dev/loop1 /dev/virtualmachines/windows7-x64
>>
>> What's the point of adding a loopback device on top of the LV? Running
>> kpartx on the LV itself will work just fine and this just adds an
>> unnecessary layer of overhead and complexity unless I am missing
>> something.
>>
>> > 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.
>>
>> Assuming that the LV given above is a whole-disk image containing a DOS
>> MBR partition table:
>>
>> dd if=/dev/virtualmachines/windows7-x64 of=/tmp.mbr.img bs=512 count=1
>>
>> You could also do the same with the loopN device that you set up
>> earlier, although I still don't see the need for that step.
>>
>> > 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
>>
>> Should work fine, just be sure to test each step so that you are
>> confident and comfortable with it before you find yourself needing to do
>> this in anger.
>>
>> Regards,
>> Bryn.
>>
>>
>> --
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>>
>
> Hi Bryn,
>
> I have just tried an experiment without the losetup step. I have verified
> that it works using kpartx only.
>
> root at sysresccd /root % kpartx -av /dev/virtualmachines/winxphome32
> add map virtualmachines-winxphome32p1 (253:22): 0 104840127 linear
> /dev/virtualmachines/winxphome32 63
> root at sysresccd /root % cd /dev/mapper
> root at sysresccd /dev/mapper % ls *winxphome32*
> virtualmachines-winxphome32  virtualmachines-winxphome32p1
> root at sysresccd /dev/mapper % ls -al *winxphome32*
> brw-rw---- 1 root disk 253,  0 2009-11-12 15:49 virtualmachines-winxphome32
> brw-rw---- 1 root disk 253, 22 2009-11-12 18:47
> virtualmachines-winxphome32p1
> root at sysresccd /dev/mapper % ls -al *winxphome32*
> brw-rw---- 1 root disk 253,  0 2009-11-12 15:49 virtualmachines-winxphome32
> brw-rw---- 1 root disk 253, 22 2009-11-12 18:47
> virtualmachines-winxphome32p1
> root at sysresccd /dev/mapper % fdisk -l virtualmachines-winxphome32
>
> Disk virtualmachines-winxphome32: 53.7 GB, 53687091200 bytes
> 255 heads, 63 sectors/track, 6527 cylinders
>
> Units = cylinders of 16065 * 512 = 8225280 bytes
> Disk identifier: 0x2b362b35
>
>
>                        Device Boot      Start         End      Blocks   Id
> System
> virtualmachines-winxphome32p1   *           1        6526    52420063+   7
> HPFS/NTFS
>
> Without the losetup and the corresponding overhead, it will speed up
> cloning of my virtual machines within LVs.
>
>
>
> --
> Mr. Teo En Ming (Zhang Enming) Dip(Mechatronics) BEng(Hons)(Mechanical
> Engineering)
> Alma Maters:
> (1) Singapore Polytechnic
> (2) National University of Singapore
> My Primary Blog: http://teo-en-ming-aka-zhang-enming.blogspot.com
> My Secondary Blog: http://enmingteo.wordpress.com
> My Youtube videos: http://www.youtube.com/user/enmingteo
> Email: space.time.universe at gmail.com
> Mobile Phone (Starhub Prepaid): +65-8369-2618
> Street: Bedok Reservoir Road
> Country: Singapore
>

Hi All,

I have performed a backup of my F11 Dom0's PV (60 GB) of 3 logical volumes
using fsarchiver with the maximum possible LZMA compression. The operation
took 4 hours with 2 compression threads. The resulting backup archive size
is 8.1 GB for a PV of 60 GB.

My hardware specs:

Intel Pentium Dual Core E6300 2.8 GHz
Intel Desktop Board DQ45CB
6 GB DDR2-800 memory

-- 
Mr. Teo En Ming (Zhang Enming) Dip(Mechatronics) BEng(Hons)(Mechanical
Engineering)
Alma Maters:
(1) Singapore Polytechnic
(2) National University of Singapore
My Primary Blog: http://teo-en-ming-aka-zhang-enming.blogspot.com
My Secondary Blog: http://enmingteo.wordpress.com
My Youtube videos: http://www.youtube.com/user/enmingteo
Email: space.time.universe at gmail.com
Mobile Phone (Starhub Prepaid): +65-8369-2618
Street: Bedok Reservoir Road
Country: Singapore
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