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