Make ZFS auto-destroy snapshots when the out of space?

[Kirk Strauser]

I found some nice scripts to regularly snapshot all the filesystems in 
my ZFS pool at 
http://www.neces.com/blog/technology/integrating-freebsd-zfs-and-periodic-snapshots-and-scrubs 
. One thing bothers me, though: I have to intentionally set how many 
months' worth of snapshots I want to keep. Too many and I run out of 
room. Too few and I lose some of the benefits of easy recovery of 
deleted data. My computer is better at bookkeeping than I am, so why not 
let it?

I'd propose standardizing on an attribute like 
org.freebsd:allowautodestroy. Modify ZFS's disk full behavior to scan 
for snapshots with that attribute set and destroy the oldest one, and 
continue until there's enough free space to complete a write requests or 
until out of "expendable" snapshots to destroy (at which time the normal 
disk full handler would run). Also run a daily periodic script to ensure 
that the free space stays below a configurable threshold each day so 
that ZFS isn't constantly butting up against completely full drives.

This would take all configuration guesswork out of the equation and 
would let me keep as many snapshots as I have space to maintain. If I 
want to extend my reach back in time, I can add another drive to the 
pool and the rest is handled automatically. At the same time, should I 
suddenly *want* to store massive amounts of new data, the snapshots can 
be easily and automatically cleared out to make room for the stuff I 
want to hold.

What do you think? It seems like this should be pretty easy to implement 
without requiring any upstream changes or new FreeBSD-only data 
structures. The whole thing could possibly be implemented in userspace, 
but I don't know that ZFS has any exception handling callbacks that 
would make it easy.

An unused resource is a wasted resource, right?