There's additional overhead on the drives so they're not presented as 930GB and part of the storage is reserved for use by the array.  I.e. to store logs, configuration data, etc...   There's also a section reserved for write testing by the array. 
Hi, i got a new PS6100 with 24 x 1TB NL-SAS HDD.The raid policy is RAID50 with 2 spares. can someone explain why the usable space is limited to 15,71 TB ? I thought that there are 2 striped RAID5 (n-1) sets on that machine if i chose RAID50. I start with 24 x 1 TB = 24 TB - 2 spares = 22 TB - 2 drives parity data (1 for each RAID5) = 20 TB HDDs for DATA Each TB HDD has about 930 GB usable space. 930 GB x 20 = 18600 GB 18 TB is not equal to 15,71 TB   What is wrong with my calculation?  
Usable Space of Euallogic RAID50