Hey there!

I'm not sure if you know, but EARS disks use 4K sectors with 512-byte emulation. So called Advanced Format as WD likes to name it. This can affect your speeds considerably for the worse when using RAID-Z, much less so for mirroring or striping.

A 4-disk RAID-Z with EARS would be quite slow. But a 5-disk RAID-Z with EARS should (in theory) be as fast as normal drives. This is a bit complicated, but the formula is:
128KiB / (number of drives - parity drives) = .... KiB <-- this number needs to be a multiple of 4KiB.

So with 5 disks in RAID-Z you get 128/ 4 = 32KiB which is perfect. But with 4 disks you get 128/3 = ~43KiB which is awful for these 4K sector disks.

These problems disappear when using mirroring or striping; or when chosing normal 512-byte sector disks like WD EADS and Samsung F2EG/F3. Generally i prefer 5400rpm disks, as with 100MB/s+ they are not slow at all but only half the heat generated and power used.

So which combinations would be good with 4K drives?
3-disk RAID-Z = 128 / 2 = 64KiB
4-disk RAID-Z2 = 128 / 2 = 64KiB
5-disk RAID-Z = 128 / 4 = 32KiB
6-disk RAID-Z2 = 128 / 4 = 32KiB
9-disk RAID-Z = 128 / 8 = 16KiB
10-disk RAID-Z2 = 128 / 8 = 16KiB

You shouldn't attempt more disks in a single vdev i think.

Also know that ZFS scales random I/O per vdev. A single RAID-Z would have random I/O capabilities of a single disk; which is not that bad as RAID5 is usually bad in random writes; this solution should be much better. But it does mean you would want multiple vdevs. So groups of 5 would be peferable to groups of 9 or 10. Also i think RAID-Z with 9 disks is not secure enough.

You *DO* need to make sure that you're using controller without RAID function; or that controller will drop your disk instead; preventing ZFS from accessing it. That's not what you want; you want a SATA HBA without RAID.