The new feature allows a ZFS user to expand the size of a single RAIDz vdev. For example, you can use the new feature to turn a three-disk RAIDz1 into a four, five, or six RAIDz1.
It’s pretty surface level only for now (for what seems like obvious reasons to me). My understanding is that disks will need to be added in parity or you lose efficiency, and data will not be redistributed across them. But you will still be able to add disks to an existing vdev, finally.
With the new code, you’ll be able to
attachnew disks to an existingRAIDzvdev as well. Doing so expands the vdev in width but does not change thevdevtype, so you can turn a six-diskRAIDz2vdev into a seven-diskRAIDz2vdev, but you can’t turn it into a seven-diskRAIDz3.
So basically, you can add disks to vdevs to address them as the same vdev, although it doesn’t alter the striping underneath. This is pretty handy if you have extra storage capacity and are OK losing the efficiency difference. In my use case, I have a 36 bay server with 12 drives in RAIDz2. This will let me add 24 drives (triple my capacity) without having to create a new vdev, even though I won’t have as much capacity as a 36 drive RAIDz2 array.