[Fedora-xen] How to backup a Domu filesystem (on LVM) from Dom0 on Fedora 8 ?

[Anne Facq]

Gerd Hoffmann wrote:
> Anne Facq wrote:
>> Because in Dom0 the device for a paravirt Domu (/dev/vg00/lvfoo in my
>> case) is mapped to a virtual disk /dev/xvda (I created the Domu with
>> virt-install), and the only way I found to mount this Domu filesystem,
>> is to :
>> - get the size of the boot sector of /dev/vg00/lvfoo
>>     (with fdisk -ul /dev/vg00/lvfoo)
>>     = 63
>> - compute the offset (multiply by block size)
>> - specify this offset to the mount command :
>> mount -o offset=32256 /dev/vg00/lvfoo /mnt/
> 
> Uh, oh, quite complicated and error prone.  Can be done much easier.
> 
> Option (1): Use kpartx, it will create device mapper mappings for your
> partitions.  Try "kpartx -v -a /dev/vg00/lvfoo", should give you
> /dev/vg00/lvfoop[1234].  "kpartx -d ... " removes the mappings.
> 
> Option (2): (works only with xen kernel): Configure the device as
> virtual disk *in Domain-0*: "xm block-attach 0 phy:/dev/vg00/lvfoo xvda
> w".  Gives you /dev/xvda[1234].  "xm block-detach ..." removes it.

Many thanks for this solution, it helps me a lot.

If this can help someone, here are the steps to backup a Domu 
(filesystem on LVM in backend and on xvda in frontend) with the dump 
command from Dom0 (Fedora 8), using one of the 2 solutions you advised, 
kpartx :

----------------------------------------------------------------

# The DomU named "foo" must be stopped before dump
xm shutdown -w foo

# Creates device mapper mappings /dev/mapper/lvfoop[1234] for 
/dev/vg00/lvfoo
kpartx -v -a /dev/vg00/lvfoo

# Mounts temporarly the filesystem to backup
mount /dev/mapper/lvfoop1 /mnt/lvfoo/

# Launches the dump
/sbin/dump 0 -L FULL_foo -f /BACKUP-XEN/foo.colddump /mnt/lvfoo/

# Umount the filesystem
umount /mnt/lvfoo/

# Removes the mappings
kpartx -d /dev/vg00/lvfoo

# Unmount the filesystem
umount /mnt/lvfoo

# The DomU can now be restarted
xm create --config /var/lib/xend/....

----------------------------------------------------------------

Regards,

Anne Facq