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

[Mr. Teo En Ming (Zhang Enming)]

On Thu, Nov 12, 2009 at 6:18 PM, Geert Janssens <info(a)kobaltwit.be&gt; 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
>
>  --
>  Kobalt W.I.T.
>  Web & Information Technology
>  Brusselsesteenweg 152
>  1850 Grimbergen
>
>  Tel  : [phone]
>  Email: info(a)kobaltwit.be
>
>  
Thank you!

I will omit the losetup step.

-- 
Mr. Teo En Ming (Zhang Enming) Dip(Mechatronics) BEng(Hons)(Mechanical
Engineering)
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(2) National University of Singapore
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