Mount disk images

Friday 18 July 2008

Submitted by Pablo Iranzo Gómez

Latest addition: Friday 18 July 2008

I’ve been using a Xen guest under RHEL 5.2 to hold this webserver, and because of failures, I choose to keep a copy of the full disk image on another machine.

Having to transfer the full disk in the network means stop the server (xen guest), rsync the image on disk (wait 40 minutes), then start guest again.

After doing the initial image transfer, it would be easier to just sync updated files, but... how to loop mount a full disk?

In my case, the hdd image contained a partition for /boot and a partition for a LVM pv.

First, I needed to check Number of cylinders in virtual disk inside Xen Guest. Using fdisk I could check that number, for example, 777.

On the remote system, the one with the full image transferred previously I could then do:

fdisk /var/lib/xen/images/GUEST.img -C 777 -l -u and will yield something like:

255 heads, 63 sectors/track, 777 cylinders, 0 sectores en total
Unidades = sectores de 1 * 512 = 512 bytes


                        Disposit. Inicio    Comienzo      Fin      Bloques  Id  Sistema
/var/lib/xen/images/GUEST.img1   *          63      208844      104391   83  Linux
/var/lib/xen/images/GUEST.img2          208845    12482504     6136830   8e  Linux LVM
In this case, as I want to access my LVM volume, so I need to convert the partition start to a size, so:

START= 512 * 208845 = 106928640

and then thanks to losetup:

libre=`losetup -f` #Get a free loop device losetup -o 106928640 $libre /var/lib/xen/images/GUEST.img #setup the device for the 2nd partition pvscan #scan for LVM’s pv vgscan # same for VG lvscan # same for LV’s lvchange -a y /dev/GUESTvg #activate LV’s on ’GUESTvg’ VG

and mount our drives doing mount /dev/GUESTvg/LVMunit where desired ;)

At this point I can just run:

rsync -avur —perms —progress —delete remoteserver:/  /mnt/DISKIMAGE

And get a ’working’ copy of the remote machine but just copying changed elements.