Re: [Xen-devel] consistent LVM snapshot of domUs from dom0

[Nikola Ciprich <extmaillist@xxxxxxxxxxx>]

Hi Mike,
I think that this problem can be solved using procedure Mark proposed - 
freeze filesystems of all running domUs using some xm command, then create 
snapshot and unfreeze them all again. I think that You won't need suspend 
at all.
We just now need to implement it :)
n.

 On Sat, 8 Dec 2007, Mike Sun wrote:

> Thanks for the question Nikola and the answer Mark, I was wondering
> the same thing.
>
> Here's another scenario.  What if dom0 has a mounted LVM filesystem,
> and a file-backed VBD located on that filesystem is used as the root
> filesystem by a domU.  Would LVM snapshots of the mounted LVM
> filesystem provide consistent checkpoints?
>
> In looking at how xm suspend works, it seems that only memory state is
> saved and nothing is done to ensure the guest domain puts its
> filesystem into a consistent state, though I'm not completely certain
> about that.
>
> Mike
>
> On Dec 8, 2007 1:26 PM, Mark Williamson <mark.williamson@xxxxxxxxxxxx> wrote:
> > > I'd like to ask a question regarding LVM snapshots:
> > > normally if You make a snapshot of mounted LVM partition, DM
> > > infrastructure handles putting fs into consistent state using VFS
> > > infrastructure. BUT, does it work even if I make snapshot of LV used by
> > > XEN?
> > If your LVM is running in dom0 and you snapshot the storage used by a domU
> > then, no, it won't snapshot the domU's filesystem automatically.
> >
> > > If not, I guess it should not be that hard to fix, I guess we just need to
> > > notify domU kernel to call needed VFS function to put fs into consistent
> > > state prior to creating snapshot.
> > Yep.
> >
> > > Can somebody comment on it, eventually correct me if I'm wrong?
> > You're quite right.  I've just had a look at it and it looks quite feasible to
> > do this.  The obvious simple way to implement it would be to add an xm
> > command that causes a domain to freeze its filesystems into a consistent
> > state and then return when that is done.
> >
> > e.g.
> >
> > xm create mydomain
> > xm fs_freeze mydomain
> > <do backup>
> > xm fs_unfreeze mydomain
> >
> > Ideally the backup operation would just take a snapshot e.g. using LVM or
> > qcow.  With a bit more tools / storage integration we could have an "xm
> > safesnapshot" command that would create a coherent snapshot of the disk.
> >
> > It would be awesome.
> >
> > Cheers,
> > Mark
> >
> > --
> > Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
> > Mark: To answer a question with a question: What use is a skateboard?
> > Dave: Skateboards have wheels.
> > Mark: My wheel has a wheel!
> >
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
--


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