Well with raidz1 you could have 3-some number...lets call it 10 drives. If one drive fails your ok. If another drives fails before the other is replaced and resilvered, your pool is gone. During the resilvering process, there is extra load on the disks so the chances of another drive failing before the previous failed drive is repaired are greater. That's why raidz2/z3 is preferred. They can handle more failures before the pool is gone. As for mirroring, that is your best bet for only two drives. Say you had 4 drives, each in a mirror to create two vdev's. You could theoretically lose two drives and be ok, as long as the two drives that failed were in different mirrors. Also...once you create a vdev, its set. You cant create a raidz vdev of 6 disks and then expand it to 8 disks...dont work like that. You could however create another raidz vdev and add it to the pool, but you cannot add an additional disk to the previous raidz vdev to increase the space. With mirrors, you could create a new mirror to increase space with only two disks. There are benefits/pitfalls to both mirrors and raidz...you need to choose what works best for what you're trying to accomplish.