Re: [Xen-users] restoring files to guest domains

[Nico Kadel-Garcia <nkadel@xxxxxxxxx>]

Jared wrote:
> On 03/05/2008 11:44 PM, Fajar A. Nugraha wrote:
> > Dude, it doesn't work that way.
> > The way I see it, you're writing data to /dev/vg1/guest-backup, but 
> > then you REMOVE the LV afterwards. And you expect it to show up on 
> > /dev/vg1/guest-disk?
> OK, so I'm officially being dense.  I can live with that.  :-)
>
> > The "read/write" part means you (should) be able to create a snapshot 
> > of an LV, and write new data to that snapshot. The data will then be 
> > available ONLY to the snapshot, not to the original LV.
> I understand what you're saying, and when dealing with normal 
> filesystem mounts I'd fully expect that behavior, but the LVM 
> snapshots are throwing me for a loop.  There are two things that made 
> me believe this would work:
> * What's the purpose of allowing write support to a snapshot if the 
> written files are not populated on the original volume?  I don't get 
> the "benefit" of being able to write to a snapshot if the changes will 
> be completely discarded.
> * Snapshots work, based on my very limited understanding at this 
> point, on essentially caching changes made to the source volume while 
> the snapshot exists, then flushing those changes to disk once the 
> snapshot is removed.  Based on that, and on the bullet above, I 
> figured there may be some "reverse" caching or something (for lack of 
> a better term) that would flush changes written to the snapshot back 
> to the source volume as well.
> > To accomplish what you're looking for, you must copy the file using 
> > either scp (or some other network-file-transfer), or shutdown the 
> > guest and mount the LV on dom0.
> That's what I'm trying to avoid doing.  Shutting down the guest is 
> definitely not an option.  I know I could do that and directly mount 
> the volume from the host pretty easily, but I don't want to interrupt 
> the services running on that guest.  scp has always been my fallback 
> option, but I'm trying to come up with a more direct solution.  Since 
> I'm working with volumes created and managed on the host, it seems 
> reasonable to assume that there should be some way to write data to 
> the volumes.
Because you can *VERY QUICKLY* make a snapshot, even of a live 
environment, and then do more painful or slower options on that snapshot 
while the live fileystem is doing whatever it needs to do. This includes 
making tape backups, analyzing the data of a database without having to 
lock it while doing your own work and having it change under you, etc.


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