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

[Nikola Ciprich <extmaillist@xxxxxxxxxxx>]

Hi Mark,
and sorry for not replying for so long.
Parallax looks really promising, and I'll surely keep eye on it...

> 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.
Well, sure I'd like to share, I didn't have much time to play with it yet, 
I've mostly studied a bit on how the DM stuff works, and I think the 
simplest, yet working solution could be:
$ xm freeze_dev xvda1 myDom
	- this will tell the domU kernel to call freeze_bdev(...) (see 
fs/buffer.c) for the given device to put it into consistent state, and 
stop using it for a while
then from dom0 You can safely create a snapshot of lv on top of domU's 
device is sitting:
$ lvcreate -s blabla

$ xm unfreeze_dev xvda1 myDom
	- this will tell the domU kernel to call thaw_bdev(..), which 
will allow domU to continue using device
What do You think about it?

> 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 :-)
Yup, agree :)
Cheers!
nik


> 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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