There might be a huge mistake when NetApp FAS system being introduced to customers.
Total number of disks = N
Total spare disks = S
Total parity disks = P
Usable space = the right-sized capacity of disks * (N – S – P) *0.9
where 0.9 is actually usable space in an NetApp FAS storage system.
The 10% of the reserved space is dedicated to kernel overhead, RAID checksum, WAFL reserve and FlexVol metadata.