|Total usable storage capacity (TB):||0.50 *|
|RAID type:||RAID 4 (stripe set with parity and dedicated parity disk)|
|Number of RAID groups:||1|
|Number of drives per RAID group:||2|
|Total number of drives:||2|
|Drive capacity (GB):||500|
|Capacity of a single RAID group (GB):||500|
|Space efficiency:||0.5 (50%)|
|Fault tolerance:||1 disk drive per RAID group|
|IO penalty (read):||1/1 (one RAID IO per each host IO)|
|IO penalty (write):||4/1 (4 RAID IOs per each host IO)|
|Minimum number of drives per RAID 4 group:||2 (in NetApp implementation)|
|Single drive cost:|
|Cost per TB usable:|
RAID 4 is not commonly used due to its write performance shortcomings. Since all parity information is written to one drive, write performance of a RAID 4 group is equivalent to performance of a single drive.
NetApp is probably the only major storage vendor that has implemented RAID 4 in its storage appliances. RAID 4 performance shortcomings are mitigated by the NetApp proprietary Write Anywhere File Layout (WAFL). WAFL never overwrites existing disk blocks and stores new data using available free space. In addition, WAFL optimizes disk write operations by caching and aggregating incoming write requests in NVRAM first, and then writing data to disks in full stripes.
* Storage reserved for WAFL operations reduces usable capacity provided by the underlying RAID group(s). For more details see NetApp Usable Space Calculator.