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

[Jared <list-xen@xxxxxxxxxxx>]

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.
> This is different from (lets say) Solaris Zones, where a non-global zone 
> filesystem is visible from the global zone. In this case you can write 
> files from the global zone and have the non-global zone see the new 
> data. I believe it's not recommended to do so, but it works, since both 
> global and none global zone can have simultaneous access to the same 
> filesystem. It won't work with Xen.
Interesting.  I can't say I have much desire to switch over to Solaris, 
though.  :-)  This isn't an absolutely "must have" feature, it'd just be 
really nice.
Thanks for the detailed response.  It definitely helps clear some things up.

--
Jared

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