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

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> 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!

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