Re: consistent LVM snapshot of domUs from dom0

[Nikola Ciprich <extmaillist@linuxbox.cz>]

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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