With RAID10, one could swap out 2 drives to get a size increase. With 2 4 disk vdevs, one could swap out 4 drives for a size increase. So I'm assuming you hav

With RAID10, one could swap out 2 drives to get a size increase. With 2 4 disk vdevs, one could swap out 4 drives for a size increase. So I'm assuming you have a single 8 disk vdev, and no spare places to put disks.