23-04-2012, 01:01 PM
RAID
RAID.pptx (Size: 156.64 KB / Downloads: 36)
RAID?
Redundant Array of Independent Disks
A set of disk stations treated as one logical station
Redundant capacity is used for parity allowing for data repair
Levels of RAID
6 levels of RAID (0-5) have been accepted by industry
Level 2 and 4 are not commercially available, they are included for clarity
RAID 0
User and system are distributed over the disks so that there is a reasonable chance for parallelism
Disk is logically a set of strips (blocks, sectors,…). Strips are numbered and assigned consecutively to the disks
RAID 1
RAID 1 does not use parity, it simply mirrors the data to obtain reliability
RAID 2
Error correction code (Hamming code) allows for correction of a single bit error
Is still expensive, only used in case many frequent errors can be expected
RAID 3
Level 2 needs log2(number of disks) parity disks
Level 3 needs only one, for one parity bit
In case one disk crashes, the data can still be reconstructed
RAID 2-3 have high data transfer times, but perform only one I/O at the time so that response times in transaction oriented environments are not so good
RAID 4
Larger strips and one parity disk
Blocks are kept on one disk, allowing for parallel access by multiple I/O requests
Parity disk may be a bottleneck
Good response times, less good transfer rates
RAID 5
Distribution of the parity strip to avoid the bottle neck.
Can use round robin
Parity disk = (-block number/4) mod 5
Alternatives to RAID
Single Large Expensive Disk (SLED)
Redundant Array of Distributed Disks (RADD)
More alternatives: 2D-RADD, C-RAID, RO WB-optimistic and RO WB-pessimistic
Advantages of RAID
Affordable alternative to mass storage
Cost/Megabyte is dropping.
Smaller drives make this possible.
Obtain performance of expensive high end disks
High throughput and reliability
Use of small diameter disks.
Seek distances lower.
High I/O rates, less power/megabyte.