Protection from disk failure is usually accomplished using RAID
Protecting Against System Failure | 263
(Redundant Array of Inexpensive Disks) technology. The use of redundant storage
has become common for systems of all sizes and types for two primary reasons: the
real threat of disk failure and the proliferation of packaged, relatively affordable
RAID solutions.
RAID technology uses one of two concepts to achieve redundancy:
Mirroring
The actual data is duplicated on another disk in the system.
Striping with parity
Data is striped on multiple disks, but instead of duplicating the data itself for
redundancy, a mathematical calculation termed parity is performed on the data
and the result is stored on another disk. You can think of parity as the sum of
the striped data. If one of the disks is lost, youcan reconstruct the data on that
disk using the surviving disks and the parity data. The lost data represents the
only unknown variable in the equation and can be derived. You can conceptualize
this as a simple formula:
A + B + C + D = E
in which A??“D are data striped across four disks and E is the parity data on a fifth
disk.
Pages:
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578