Re: [Xen-users] Xen Backup Method

[Kevin Fox <Kevin.Fox@xxxxxxx>]

On Tue, 2009-04-07 at 12:30 -0700, Javier Guerra wrote:
> On Tue, Apr 7, 2009 at 1:46 PM, Kevin Fox <Kevin.Fox@xxxxxxx> wrote:
> > Has anyone tried using 2.6.29 file system freezing with lvm
> snapshotting
> > yet? That should solve the possible corrupt filesystem problem.
> 
> sounds promising.  but:
> 
> 1: its done from 'inside' the VM, so it might allow better network
> backup systems but not replace them.

So, in a single kernel (2.6.29+), lvm can tell the file system using the
block device to freeze so it can take its snapshot, and then tell it to
unfreeze. How hard would this be to extend this so that an LVM sitting
in Dom0 could forward the freeze calls to DomU? Say, through the virtual
block devices. You could then run your backups in Dom0 instead of every
DomU. This would save you on per client licensing fees.

> 2: it does nothing about complex applications with their own caches
> and file structures: IOW, databases.  if you get an 'in VM' snapshot
> capability, you still have to flush caches and suspend operations for
> the snapshot.

A lot of backup systems I've seen tough, largely just save the files, or
ignore them if they are written while saving. They don't bother to flush
caches and suspend. It doesn't work for everything (databases), but for
most applications it is fine. One example use case would be each users
desktop session gets a virtual machine. You could backup the users data
without needing to have a backup client in the vm.

Kevin
> 
> --
> Javier
> 
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> 
> 


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