Raidz-1 parity disks?

Question

Duplicate on Server Fault:

Raidz-1 parity disks?

I am a bit confused about how to configure my zfs pool. What is the exact difference between adding 8 disks to a raidz1 pool or adding 2x4 to the pool with 2 raidz1 groups. Example

zpool create testPool raidz1 d1 d2 d3 d4 raidz1 d5 d6 d7 d8

zpool status shows the same amount of usable disk space but when running zfs list testPool the one with the 2 raidz groups shows 1/2 of the usable diskspace of the one with all disks in one group.

Does this method define 2 parity disks? How are the number of parity disks defined? If I create the following pool with raidz2 it shows pretty much the same usable space.

zpool create testPool raidz2 d1 d2 d3 d4 d5 d6 d7 d8

Any hints on this?

Thanks a lot