Fedora xen - Jun 2008 - Subject: Re: How to backup a Domu filesystem (on LVM) from Dom0 on Fedora 8 ?

This helped me a great deal. Thanks. I just have a few questions...

1) is it best to unmount the /mnt/lvfoo/ before or after kpartx -d?
because you have it in your procedure list both before and after
kpartx -d.
2) is it possible to explore this dump file by either mounting it or
some other method?
3) what is the best procedure to replace this dump file in case of a
disk failure? i.e. just install xen and use virt-install?
4) is this the best possible method of backing up a domU?

Thanks.

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

Henok Mikre wrote:
> This helped me a great deal. Thanks. I just have a few questions...
> 
> 1) is it best to unmount the /mnt/lvfoo/ before or after kpartx -d? because
you have it in your procedure list both before and after kpartx -d.
The /mnt/lvfoo must be unmounted before the kpartx -d, otherwise
it will generate problems on the filesystem of the DomU (because the 
kpartx -d deletes partition mappings)

But the line "umount /mnt/lvfoo" after "kpartx -d" is an
error due to a
"copy and paste", sorry.

So 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) :

------------------------------------------------------------
# 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

# The DomU can now be restarted
xm create --config /var/lib/xend/....
------------------------------------------------------------
> 2) is it possible to explore this dump file by either mounting it or some
other method?
You can explore the dump file, using the "restore" command like this
restore -if /var/BACKUP-XEN/foo.colddump

You can also restore in another filesystem the filesystem (of the Domu) 
that you backuped, following these steps :

lvcreate -L2048 -n lvfoo2 vg00
mkfs.ext3 /dev/vg00/lvfoo2
mkdir /mnt/lvfoo2
mount /dev/vg00/lvfoo2 /mnt/lvvfoo2
cd /mnt/lvfoo2
restore -rf /BACKUP-XEN/foo.colddump
> 4) is this the best possible method of backing up a domU?
This method works well, but I would be interested in knowing other 
methods :-)

Regards

Anne Facq