new unionfs & moving files around

This is the situation:
	- there's one file system on a RAID controller, somewhat slow and with 
limited space, let's call it "production"
	- there's another file system on a big IDE drive
	- a lot of files are saved on the production file system, but the 
usefulness of that data shaply declines with time (recent files 
absolutely need to be kept, but older files are optional)

The idea:
	- create a unionfs that covers both file systems, so that new files are 
created on the RAID-backed file system, but all files are visible
	- create a cron job that moves older files from the production file 
system to the spare file system

I've not started it yet (waiting for csup to RELENG_6) but is there any 
"hidden" reason this shouldn't work? I'm asking because the files will 
be moved from one part of the unionfs (upper) to the other (lower), on 
different file systems, probably while concurrently accessed.