TYPE OF RAID :)

RAID

• Redundant Array of Independent Disks 

• Redundant Array of Inexpensive Disks 

• 7 levels numbered 0 through 6 – does not imply a hierarchical relationship 

• Set of physical disk drives viewed by the OS as a single logical drive 

• Data are distributed across the physical drives of an array 

• Redundant disk capacity used to store parity information – guarantees data 

recoverable

RAID Level 0 - Striping 

• No redundancy 
• Data striped across all disks 
• Uses Round Robin striping

• Increase speed 
   o Multiple data requests found on different disks 
   o Multiple disks can be seeked in parallel 
   o A set of data is likely to be striped across multiple disks 

RAID Level 1 - Mirroring 

• Mirrored disks 
• Data is striped across disks
• 2 copies of each stripe exist on 2 separate disks 
• Read request handled by either disk – minimum seek plus rotation latency 
• Write requests go to both disks 
• Recovery from failure is simple – swap out bad disk and re-mirror 
• Expensive – requires twice the disk space 



RAID Level 2 – Parallel access 

• Disks are synchronized – heads in same position on every disk 
• Stripes are very small – single byte or word 
• Error-correcting code calculated across corresponding bits on each disk 
• Multiple parity disks store the code in corresponding positions – Hamming code 
• Too much redundancy – expensive – overkill so not implemented 




RAID Level 3 – Parallel access 

• Similar to RAID 2 
• Only a single redundant disk – no matter what the array size is 
• Simple parity bit is computed for the set of individual bits in the same position 
• Data on a failed disk reconstructed from surviving data and parity info 
• Very high transfer rates – small stripes yield parallel transfer from all disks 



RAID Level 4 – Independent access 

• Each disk operates independently 
• High I/O request rates – separate I/O request satisfied in parallel 
• Large stripes are used 
• Bit-by-bit parity strip is calculated across corresponding strips on each disk 
• Parity strips stored in the corresponding strip on the parity disk 
• Every write must involve a parity disk – potential I/O bottleneck


RAID Level 5 – Independent access 

• Similar to RAID 4 
• Distributes the parity strips across all disks 
• Uses Round Robin allocation scheme 
• Avoids RAID 4 I/O bottleneck 
• Commonly used in network servers 


RAID Level 6 – Independent access 

• Two parity calculations 
• Stored in separate blocks on different disks 
• Requires N+2 disks – where N is number of disk required for user data 
• Extremely high data availability 
o Three disk must fail before loss of data 
o Substantial write penalty – affects two parity blocks 


posted by; Hazyanty Shahira Abd. Halim