Compaq 3200 manual Data Guarding RAID, E-8 Understanding Drive Arrays

E-8Understanding Drive Arrays

Data Guarding (RAID 4)

Data guarding, also called RAID 4, assures data reliability while using only a small percentage of the logical drive storage capacity. A designated, single drive contains parity data. If a drive fails, the controller uses the data on the parity drive and the data on the remaining drives to reconstruct data from the failed drive. This allows the system to continue operating with slightly reduced performance until you replace the failed drive.

Data guarding requires a minimum of three drives (two data drives and one parity drive) in an array and allows a maximum of 30 drives (29 data drives and one parity drive). For example, in an array containing three physical drives, data guarding uses only 33 percent of the total logical drive storage capacity for fault tolerance. A 14-drive configuration (13 data drives, one parity drive) uses only 7 percent; a 30-drive configuration (29 data drives, one parity drive) uses only 3 percent.

NOTE: Given the reliability of a particular generation of hard drive technology, the probability of an array experiencing a drive failure increases with the number of drives in an array. Compaq recommends the number of drives in an array be limited to 14.

DATA DATA DATA PARITY

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P = Parity

WAR2-052.AI, 9-2.EPS

Figure E-6. Data guarding stores redundant data [P] on a separate drive.