possible 3rd solution (as mentioned by SirMaster in this FreeNAS forum post) :
- add new disk(s) to zpool
- copy many files to another new /mnt/pool/temp/ directory
- delete original files:
rm -rf original/
- rename dir back:
mv temp/ original/
This works because ZFS will proportionally place writes to whichever vdev has most free space, in this case the brand new drives which were empty. (as of 0.7, zfs will favor faster drives for writes, but lets assume your 2 new drives are same or greater performance than the original drives)
It's probably slower than
zfs send | zfs recv, but simpler because you don't have to create/destroy snapshots.
You can run
zpool list -v before and after to see each vdev utilization.
Also, found a php script that does the copy/delete/rename procedure on a file-by-file basis. This was linked in an answer from a similar (but zvol) question a few years older. (didn't test this script personally)