[linux-lvm] preventing snapshots from running out of room.

[Dan Merillat]

In the project I'm working on, snapshots as backups are more critical
then the "live" data.  What I'd like is the original volume to get write
errors if I run out of snapshot room (with the corresponding printk
causing my log-watcher to lvextend the snapshot and re-activate the
original)  Obviously, this would be a panic situation.  But the reasons
for this are many:  If I'm doing something really dangerous on my
database, I'd shutdown or quiesce it, snapshot, and activate it again.
Do the dangerous merge, and drop the snapshot if everything worked out.

If I miscalculate how much disk space would be needed, what could happen
is I screwup the master, AND the snapshot breaks.  I'd much rather the
master was stuck in disk-wait waiting for a free COW block.

I have not yet looked into LVM2/device-mapper, so if it handles this
situation I'll just upgrade to that instead.

P.S.: I know you can always allocate as much snapshot space as the
original, but when you're talking about a 250gig filesystem it's not
always possible to have a spare quarter terr on the same machine.

--Dan
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