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

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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