Possibility to have a "transient" snapshot?

[James West <james@terminalsystems.com>]

This is just a random idea that popped through my mind while I was 
looking into hardening a filesystem against damage, might be 
impractical, but the idea seems promising, and well suited to a snapshot 
file system.

I'm sure some creative shell scripting could do something like this 
already, but I was more looking for something more bulletproof.

General idea would be to have a transient snapshot (optional quota 
support possibility here) on top of a base snapshot (possibly readonly). 
On system start/restart (whether clean or dirty), the transient snapshot 
would be flushed, and the system would restart the snapshot, basically 
restarting from the base snapshot. If desired, the transient snapshot 
could be promoted to a regular snapshot (say after a software upgrade). 
If desired, a different base snapshot could be selected (although I'm 
sure the file system would have to be restarted to do this)

 From a caching perspective, this could make a noticable performance 
difference, since if you're running in a transient snapshot, the file 
system can be _extremely_ lazy about committing changes to disk.

For the optional quote support I mentioned, on an unattended box, if the 
quota gets exceeded, a system reboot would probably fully correct the 
system. (Presumably a log file got out of control in that situation).