Re: consistent LVM snapshot of domUs from dom0

[Nikola Ciprich <extmaillist@linuxbox.cz>]

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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