OpenZFS won't let you alter the array geometry, so if you start with an 8-drive RAID-Z2 pool, you're stuck with it. You can expand the capacity of the pool by

OpenZFS won't let you alter the array geometry, so if you start with an 8-drive RAID-Z2 pool, you're stuck with it. You can expand the capacity of the pool by replacing each drive, changing them one by one and re-synchronizing the array between replacements.