Group your disks in bunches of 4 or 5 per Raidz, no more. And have them on the same controller or SAS-expander per bunch. Use striping over the bunches. Don't use hotspares, for performance maybe avoid RAIDz6. Try out and benchmark a lot. Get more RAM, lots more RAM.
I think the optimal number of RAIDz5 disks is 3, if you just want performance. But this wastes lots of space of course. Also, the number of SAS/SATA-channels per controller and the topology of expanders is important. Thats why I don't think there is a recipy, you have to try it out for each new kind of hardware.
And as another thread pointed out, stripe size is also an important parameter.
Yes. RAIDz (without the "1") was the original RAID5-equivalent, RAIDz2 is equivalent to RAID6. However since nobody really knows what the hell z1 and z2 is and z1 is easy to mix up with RAID1 for nonZFS people, calling it z5 and z6 is far less confusing.
It's the number of parity disks, pretty simple. There has been occasional talk of making the number arbitrary, though presently only raidz1, raidz2, and raidz3 exist.
