How to Backup and Restore MBR within Logical Volumes?

Thu Nov 12 10:55:12 UTC 2009

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
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