soft raid5 on OES linux sp2

Hello all,

I recently installed OES SP2 with linux kernel. The system is on
hardware RAID1 device built from two disks, which works fine. Due to
hardware limitations, the data storage needs software RAID5. I created
/dev/md0 from the other three disks with mdadm under linux, but I cannot
create nss pools on it neither with iManager nor with nssmu. Is this
approach totally wrong, or I miss something trivial?
Another question: How can I tune the parameters (for example cluster
size) of an NSS partition?
Thank you!