Appendix C. Terminology 783
Real-time Compression
Real-time Compression is an IBM integrated software function for storage space efficiency. 
The RACE engine compresses data on volumes in real time with minimal effect on 
performance.
Redundant Array of Independent Disks
RAID refers to two or more physical disk drives that are combined in an array in a certain way, 
which incorporates a RAID level for failure protection or better performance. The most 
common RAID levels are 0, 1, 5, 6, and 10. Some storage administrators refer to the RAID 
group as TRAID - Traditional RAID.
RAID 0
RAID 0 is a data striping technique that is used across an array and no data protection is 
provided.
RAID 1
RAID 1 is a mirroring technique that is used on a storage array in which two or more identical 
copies of data are maintained on separate mirrored disks.
RAID 10
RAID 10 is a combination of a RAID 0 stripe that is mirrored (RAID 1). Therefore, two identical 
copies of striped data exist, with no parity.
RAID 5
RAID 5 is an array that has a data stripe, which includes a single logical parity drive. The 
parity check data is distributed across all the disks of the array.
RAID 6
RAID 6 is a RAID level that has two logical parity drives per stripe, which are calculated with 
different algorithms. Therefore, this level can continue to process read and write requests to 
all of the array’s virtual disks in the presence of two concurrent disk failures.
Read intensive drives
The read intensive flash drives that are available on IBM Storwize V7000 Gen2, IBM Storwize 
V5000 Gen2, and IBM SAN Volume Controller 2145-DH8, SV1, and 24F enclosures are one 
Drive Write Per Day (DWPD) Read Intensive drives. 
Rebuild area
Reserved capacity that is distributed across all drives in a redundant array of drives. If a drive 
in the array fails, the lost array data is systematically restored into the reserved capacity, 
returning redundancy to the array. The duration of the restoration process is minimized 
because all drive members simultaneously participate in restoring the data. See also 
“Distributed RAID or DRAID” on page 775.
Reclaimable (or reclaimed) capacity
Reclaimable Data is the capacity that is no longer needed. Reclaimable capacity is created 
when data is overwritten and the new data is stored in a new location, when data is marked as 
unneeded by a host using the SCSI unmap command, or when a volume is deleted.