RAID

• Redundant Array of Independent Disks


• Redundant Array of Inexpensive Disks


• 6 levels in common use


• Not a hierarchy


• Set of physical disks viewed as single logical drive by O/S


• Data distributed across physical drives


• Can use redundant capacity to store parity information

RAID 0

• No redundancy


• Data striped across all disks


• Round Robin striping


• Increase speed

—   Multiple data requests probably not on same disk

—   Disks seek in parallel

—   A set of data is likely to be striped across multiple disks

RAID 1

• Mirrored Disks


• Data is striped across disks


• 2 copies of each stripe on separate disks


• Read from either


• Write to both


• Recovery is simple

—   Swap faulty disk & re-mirror

—   No down time

• Expensive

RAID 2

• Disks are synchronized


• Very small stripes

—   Often single byte/word

• Error correction calculated across corresponding bits on disks


• Multiple parity disks store Hamming code error correction in corresponding positions


• Lots of redundancy

—   Expensive

—   Not used

RAID 3

• Similar to RAID 2


• Only one redundant disk, no matter how large the array


• Simple parity bit for each set of corresponding bits


• Data on failed drive can be reconstructed from surviving data and parity info


• Very high transfer rates

RAID 4

• Each disk operates independently


• Good for high I/O request rate


• Large stripes


• Bit by bit parity calculated across stripes on each disk


• Parity stored on parity disk

RAID 5

• Like RAID 4


• Parity striped across all disks


• Round robin allocation for parity stripe


• Avoids RAID 4 bottleneck at parity disk


• Commonly used in network servers

RAID 6

• Two parity calculations


• Stored in separate blocks on different disks


• User requirement of N disks needs N+2


• High data availability

—   Three disks need to fail for data loss

—   Significant write penalty