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

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> 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.

There's some discussion of its current status here: 
http://www.xen.org/files/xensummit_fall07/20_DutchMeyer.pdf

It's essentially a distributed block store that's specifically oriented to the 
needs of storing VM disk images.

> hopefully I'll manage to create a fix Mark proposed...

I've been having a look at this too.  Would you like to look into sharing the 
work?  I'd got as far as plumbing some control plane stuff through xm, Xend, 
etc to talk to a stub driver in drivers/xen/fssnap/fssnap.c.  I've also 
identified where I think we'd need to hook in to get the kernel to freeze its 
filesystem.  I've not tied it all together yet though.

Once this feature is available, it'd be cool to get it plugged into various 
other layers of tools - there's lots of potential to do interesting things 
with it.  e.g.
1) manual backups
2) integrate automatically with block device snapshots
3) enable introspection of a domain filesystem e.g. filesystem contents, free 
space, checksum files, measure fragmentation, etc - all these can be 
performed on the consistent snapshot whilst the domain itself continues to 
run on the primary copy
4) etc etc etc

Lots of fun stuff :-)

Cheers,
Mark

> 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



-- 
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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