Xen users - Oct 2008 - Using dd and gzip to backup LVM backed domUs

I''m trying to come up with the best way to backup my xen domUs.  I
have a logical volume containing an ext3 root filesystem for each
domU.  Basically, I want to back up the whole filesystem in such a way
that I could restore the image on another xen server in the case of
hardware failure.   My idea so far is to back up the logical volume
with a command of this form (from the dom0):

# dd if=/dev/VolGroup00/XenImage bs=64k | gzip >
/mnt/archive/XenImage_20081017.gz

And to restore using a command of this form:

# zcat /mnt/archive/XenImage_20081017.gz | dd of=/dev/VolGroup00/XenImage bs=64k

I think this should work in a pinch but there are at least two issues
that I''d like to sort out.

First, one of the reasons I''m using gzip is because I have a lot of
free space on my domU logical volume, and I want to compress all that
unused space down to (almost) nothing when I make my backup image.  At
first, this unused space would be all zeros and would therefore
compress very nicely.  But as I understand it, if a file is deleted,
the data remains on the block device.  The blocks are no longer
allocated by the filesystem, but when I try to compress them, they
won''t compress as well as the unused blocks that are all zeros.  What
I foresee is the compressed image growing in size even though the
space used by the domU''s filesystem remains relatively steady.  One
idea would be to periodically zero out the unused blocks by filling up
the volume with a file of all zeros with something like this:

# dd if=/dev/zero of=foo ; rm foo

But I think it would be a bad idea to do this on a mounted root
filesystem.  So am I right to worry about this, and if so, what should
I do about it?

The other problem with my backup method is that it only works if the
domU is shut down.  If the domU is running, the filesystem is backed
up in an inconsistent state.  I''ve thought about using LVM snapshots
for hot backups, but if I take a backup from a snapshot of a running
system, would I be able to start up my domU from that volume without
losing data?  Or would I have to do something like save a checkpoint
(using ''xm save''), make a snapshot of the logical volume,
restore the
DomU, dd the snapshot, and then remove the snapshot?

Also, any other ideas for doing what I''m trying to do would also be
appreciated.

Thanks,

--
Agent Rooker

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

On Fri, Oct 17, 2008 at 4:30 PM, Agent Rooker <agentrooker@gmail.com>
wrote:
> I''m trying to come up with the best way to backup my xen domUs.  I
> have a logical volume containing an ext3 root filesystem for each
> domU.  Basically, I want to back up the whole filesystem in such a way
> that I could restore the image on another xen server in the case of
> hardware failure.   My idea so far is to back up the logical volume
> with a command of this form (from the dom0):
as always, it''s much better to do a backup from the DomU itself.

-- 
Javier

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

I''ve read that there can be problems with MySQL databases (for
instance) unless you do FLUSH TABLES WITH READ LOCK; before creating
the snapshot, but this would not be a problem if you used this method
to do a cold backup (i.e., creating an LVM snapshot while the domU is
shut down).

Now I''m wondering if it would be best possible to take an LVM snapshot
and a xen checkpoint at the same time so that if the domU does need to
be restored, the data not committed to disk would still be in RAM
(loaded from the checkpoint file), so there would not be any loss of
data upon restore.

Also, if someone could explain to me the dangers of hot backups with
LVM snapshots, or point me to a website explaining it, that would be
very helpful.

Thanks again.


On Fri, Oct 17, 2008 at 4:40 PM, Javier Guerra <javier@guerrag.com>
wrote:
> On Fri, Oct 17, 2008 at 4:30 PM, Agent Rooker <agentrooker@gmail.com>
wrote:
>> I''m trying to come up with the best way to backup my xen
domUs.  I
>> have a logical volume containing an ext3 root filesystem for each
>> domU.  Basically, I want to back up the whole filesystem in such a way
>> that I could restore the image on another xen server in the case of
>> hardware failure.   My idea so far is to back up the logical volume
>> with a command of this form (from the dom0):
>
> as always, it''s much better to do a backup from the DomU itself.
>
> --
> Javier
>


-- 
Agent Rooker

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users

Agent Rooker schrieb:
> I''m trying to come up with the best way to backup my xen domUs.  I
> have a logical volume containing an ext3 root filesystem for each
> domU.  Basically, I want to back up the whole filesystem in such a way
> that I could restore the image on another xen server in the case of
> hardware failure.   My idea so far is to back up the logical volume
> with a command of this form (from the dom0):
Unless you have Windows domains, the easiest approach would be to simply 
copy the files.

cp, tar, rsync - pick the one you like.

If you want a good backup system, try BackupPC - http://backuppc.sf.net


-- 
Tomasz Chmielewski
http://wpkg.org

_______________________________________________
Xen-users mailing list
Xen-users@lists.xensource.com
http://lists.xensource.com/xen-users