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

[Nikola Ciprich <extmaillist@xxxxxxxxxxx>]

Hi,
I don't know anything about parallaxfs (and neither does google it seems), 
but I guess the problem will be exactly the same. If it won't give the 
domU command to put fs into consistent state, snapshot won't be 
consistent.
hopefully I'll manage to create a fix Mark proposed...
cheers
n.

 On Sat, 15 Dec 2007, tgh wrote:

> hi
> if we use the ParallaxFS to do the filesystem snapshot, does it have the 
> similar problem ,or not? could you clear me?
> Thanks in advance
>
>
> Mark Williamson ??:
> > > 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
--


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