One of the questions that comes up time and time again about ZFS is “how can I migrate my data to a pool on a few of my disks, then add the rest of the disks afterward?”

If you just want to get the data moved and don’t care about balance, you can just copy the data over, then add the new disks and be done with it. But, it won’t be distributed evenly over the vdevs in your pool.

Don’t fret, though, it’s actually pretty easy to rebalance mirrors. In the following example, we’ll assume you’ve got four disks in a RAID array on an old machine, and two disks available to copy the data to in the short term.