Re: [Xen-users] Xen backups using LVM Snapshots

[Kevin Fox <Kevin.Fox@xxxxxxx>]

On Fri, 2008-10-17 at 10:50 -0700, John Haxby wrote:
> Kevin Fox wrote:
> > If there is such a connection between the LVM snapshotter and the
> file
> > system, I could see it not working properly if the LVM snapshotter
> is in
> > dom0 and the file system in domU. Perhaps a xen extension could be
> added
> > to support that?
> >  
> There is indeed such a connection within the same OS and yes, if you
> want to take a back up of the guest's file system you need to enlist
> its
> cooperation to get the file system into a consistent state before you
> take the snapshot and, of course, to let it go afterwards.

Yeah. I wouldn't think that would be too difficult to do. Though usually
you see calls going from DomU->Xen, not the other way around. I'm not
sure there is a mechanism in place to go the other way.

>    In general,
> you'll also need the cooperation of any applications that are running
> as
> well.

Depending on what your trying to backup, true.

> 
> In the end, though you have to ask yourself if it's worth it.   For
> example, someone, somewhere mentioned running mysqlhotbackup in the
> guest and putting the files somewhere that dom0 could see them.   Is
> it
> quicker, in the end, to recover the OS data from a backup or just run
> a
> kickstart installation to rebuild it?   Chances are that if you're
> backing up an entire system by, basically, photocopying the disk,
> you're
> backing up a load of stuff you don't need to.

I think whether its worth it all depends what kind of backups your
trying to do. If your trying to backup database servers, its probably
not.

One case where it could be worth it is if you had a virtual machine per
user for doing something like a terminal server. Each user gets their
own VM to do email, web surfing, office, etc. Being able to run one
piece of backup software on the dom0 would be less maintenance then
maintaining one copy per virtual machine. If your paying for backup
licenses, it can be substantially cheaper too.

Kevin
> 
> There are hardware configurations where dom0 driven backups are
> better,
> but to make them as good as bare-metal backups then you really need
> the
> cooperation and, in many cases, active help from the domU.
> 
> jch
> 
> 


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