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

[Dan Nelson]

In the last episode (May 29), Kirk Strauser said:
> 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.

If the kernel does the snapshot deleting itself, why not add a pool-level
property that sets the amount of free space at which the deletion starts? 
That way you don't need the cleanup script.  Alternatively, make the
org.freebsd:allowautodestroy property hold the trigger freespace amount. 
That way you can have monthly/daily/hourly snapshots but set it so the
hourly ones disappear first, then the dailies (by setting the destroy
trigger slightly higher for the ones you want to expire first).

-- 
	Dan Nelson
	dnelson at allantgroup.com