preventing snapshots from running out of room.

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