If you have let Data OnTap configure your initial aggregate (AGGR0), it will most likely have only 3 disks in it. Should any of the disks fail the system will pick up a spare and begin to rebuild the data on it. The assumption is that you have allowed this aggregate to be created as RAID-DP, in this case you actually have 2 parity drives and can sustain a double disk failure on the system.
To verify what you are looking at, from a command line run "sysconfig -r". This will list all of the RAID groups for you. I have done that on a system and the output of the system is:
Aggregate aggr0 (online, raid_dp) (block checksums)
Plex /aggr0/plex0 (online, normal, active)
RAID group /aggr0/plex0/rg0 (normal)
RAID Disk Device HA SHELF BAY CHAN Pool Type RPM Used (MB/blks) Phys (MB/blks)
--------- ------ ------------- ---- ---- ---- ----- -------------- --------------
dparity 0c.19 0c 1 3 FC:B - FCAL 10000 272000/557056000 280104/573653840
parity 0b.32 0b 2 0 FC:A - FCAL 10000 272000/557056000 280104/573653840
data 0c.22 0c 1 6 FC:B - FCAL 10000 272000/557056000 280104/573653840
I hope that this answers your question.
TC Labby