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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