<zfs nerd post>
At home (on this server) I have a Zpool consisting of 3 disks in a RAIDZ1 group. I needed more disc space, so 1 by 1 I changed my 3 x 500G drives to 1tb drives. This would of been pretty much no down time (except for an export and import of the pool) if my hardware had hot swapable drive - The cheap Dell SC430 doesen't do that unfortunately.
It took around 8 hours each for the first two discs - But the last one took over 20 hours! No idea why - My system was idle with just the resilvering taking place. If you have had experiences with this leave me a comment with how long this process took you.
SO my pool 'dump' went from
dump 921G 37K 112G 1% /dump
dump 1.8T 37K 1.1T 1% /dump
If you want to ever do this process, ZFS (once all the discs are the same size) just sees that and grows. All you need to do is replace your discs 1 by and and let them rebuild. No mucking around moving data. The best part is your Data is still avaliable, during the whole process, all be it slow to access but if you need a something you can get it. 🙂
</zfs nerd post>