http://blog.delphix.com/matt/2014/06/06/zfs-stripe-width/
TL;DR: Choose a RAID-Z stripe width based on your IOPS needs and the amount of space you are willing to devote to parity information. If you need more IOPS, use fewer disks per stripe. If you need more usable space, use more disks per stripe. Trying to optimize your RAID-Z stripe width based on exact numbers is irrelevant in nearly all cases.
For best reliability, use more parity (e.g. RAIDZ3 instead of RAIDZ1), and architect your groups to match your storage hardware. E.g, if you have 10 shelves of 24 disks each, you could use 24 RAIDZ3 groups, each with 10 disks – one from each shelf. This can tolerate any 3 whole shelves dying (or any 1 whole shelf dying plus any 2 other disks dying).
For best space efficiency, use a large number of disks in each RAID-Z group. Wider stripes never hurts space efficiency. (In certain exceptional cases, use at least 5, 6, or 11 disks (for RAIDZ-1, 2, or 3 respectively) – see below for more details.) When trading off between these concerns, it is useful to know how much it helps to vary the parameters.
Even niets...