Hardware Reference
In-Depth Information
times more likely to experience a drive failure than if you used just a single larger drive.
And because JBOD does not use striping, performance would be no better than a single
driveeither.Toimprovebothreliabilityandperformance,theBerkeleyscientistsproposed
six levels (corresponding to different methods) of RAID. These levels provide varying
emphasis on fault tolerance (reliability), storage capacity, performance, or a combination
of the three.
Althoughitnolongerexists,anorganizationcalledtheRAIDAdvisoryBoard(RAB)was
formed in July 1992 to standardize, classify, and educate on the subject of RAID. The
RAB developed specifications for RAID, a conformance program for the various RAID
levels, and a classification program for RAID hardware.
The RAID Advisory Board defined seven standard RAID levels, called RAID 0-6. Most
RAIDcontrollersalsoimplementaRAID0+1combination,whichisusuallycalledRAID
10. The levels are as follows:
•
RAID Level 0: Striping
—Filedataiswrittensimultaneouslytomultipledrivesinthe
array, which act as a single larger drive. This offers high read/write performance but
low reliability. Requires a minimum of two drives to implement.
•
RAID Level 1: Mirroring
—Data written to one drive is duplicated on another,
providing excellent fault tolerance (if one drive fails, the other is used and no is data
lost) but no real increase in performance as compared to a single drive. Requires a
minimum of two drives to implement (same capacity as one drive).
•
RAID Level 2: Bit-level ECC
—Data is split one bit at a time across multiple drives,
anderrorcorrectioncodes(ECCs)arewrittentootherdrives.Thisisintendedforstor-
age devices that do not incorporate ECC internally. (All SCSI and ATA drives have
internalECC.)It'sastandardthattheoreticallyprovideshighdatarateswithgoodfault
tolerance, but seven or more drives are required for greater than 50% efficiency, and
no commercial RAID 2 controllers or drives without ECC are available.
•
RAID Level 3: Striped with parity
—Combines RAID Level 0 striping with an ad-
ditional drive used for parity information. This RAID level is really an adaptation of
RAID Level 0 that sacrifices some capacity, for the same number of drives. However,
italsoachievesahighlevelofdataintegrityorfaulttolerancebecausedatausuallycan
be rebuilt if one drive fails. Requires a minimum of three drives to implement (two or
more for data and one for parity).
•
RAID Level 4: Blocked data with parity
—Similar to RAID 3 except data is written
inlargerblockstotheindependent drives,offeringfasterreadperformance withlarger
files. Requires a minimum of three drives to implement (two or more for data and one
for parity).
