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

[tgh <tianguanhua@xxxxxxxxxx>]

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 åé:
> > 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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