Re: [Xen-users] How to Backup and Restore MBR within Logical Volumes?

On Thu, Nov 12, 2009 at 6:18 PM, Geert Janssens <info@xxxxxxxxxxxx> wrote:

On Thursday 12 November 2009, you wrote:
dd if=/dev/virtualmachines/windows7-x64 of=mbr.w7-x64 bs=512 count=1

I think if you do this, you are only backing up the first 512 bytes of
the
logical volume, not the MBR.

Someone correct me if I am wrong.

I did some tests just to be sure. As far as I can tell, dd interacts with
lvm
in exactly the same way as with a physical disk or a loop device.

In the test I copied the first sector directly from the lvm partition or
via
the loop device. It results in exactly the same sector being copied.

Also, if you try fdisk -l on the lvm disk or the loop device, it results in
the same output.

Below is the output from my tests:

[root@aragorn:~]# losetup /dev/loop1 /dev/base/kobaltwit_f11_disk

[root@aragorn:~]# fdisk -l /dev/base/kobaltwit_f11_disk

Disk /dev/base/kobaltwit_f11_disk: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id
System
/dev/base/kobaltwit_f11_disk1 * 1 1305 10482381 83
Linux
[root@aragorn:~]# fdisk -l /dev/base/kobaltwit_f11_disk

Disk /dev/base/kobaltwit_f11_disk: 10.7 GB, 10737418240 bytes
255 heads, 63 sectors/track, 1305 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device Boot Start End Blocks Id
System
/dev/base/kobaltwit_f11_disk1 * 1 1305 10482381 83
Linux

[root@aragorn:~]# dd if=/dev/base/kobaltwit_f11_disk of=mbr.lvm bs=512
count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 7.6e-05 seconds, 6.7 MB/s

[root@aragorn:~]# dd if=/dev/loop1 of=mbr.loop bs=512 count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.000151 seconds, 3.4 MB/s

[root@aragorn:~]# diff mbr.l*
[root@aragorn:~]#

These test seem to indicate to me that the lvm layer in completely
transparent
to userland tools such as fdisk or dd.

So I still think the losetup step is superfluous and possibly causing
unnecessary overhead.

Geert

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Thank you!

I will omit the losetup step.

--
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