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

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> 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?

I believe the filesystem on the LVM volume should be consistent (as long as 
you're using one of the common Linux filesystems which can be frozen for a 
snapshot to take place).  However, the guest filesystem in the file-backed 
VBD will not be consistent: if you try to mount that VBD it will be in an 
unclean and possibly inconsistent state.

The only way you can guarantee a consistent view of a guest filesystem is with 
the guest's co-operation, either by using a network or cluster filesystem, or 
(as Nik has suggested) by explicitly asking for the domain to freeze its 
filesystem state for a while.

Cheers,
Mark

> 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



-- 
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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