I had an existing RAIDZ2 pool consisting of 4 12TB IronWolf disks. I decided to order 3 new disks, use two of them to expand the pool and keep one as a backup, even though I hadn’t even reached 10% used capacity yet. I went in and added the first disk. Then, wondering whether I could queue the expansion with the next disk, I went into the same dialog and added the next since it seemed like it would take some time. It looked like that was fine. No errors, it just accepted the expansion.
When I checked this morning it had finished expanding the pool. The VDEV now had all 6 devices, but the Usable Capacity was listed as 31.59 TiB. Each disk is 10.91 TiB, so that seems to have added only one disk of usable capacity.
I did not expect to be able to fully utilize the storage of both disks given the caveat of expanding a RAIDZ2 array, but I expected to see more than a little over 30 TiB. Was it a mistake to request the second expansion before the first one had finished?