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RAID Array Controller 2-17
Compaq Confidential – Need to Know Required
Writer: Bob Young Project: RAID Array 3000 Pedestal Storage Subsystem Hardware User’s Guide Comments:
Part Number: EK-SMCPO-UG. C01 File Name: c-ch2 RAID Array Controller.doc Last Saved On: 12/4/00 1:51 PM
RAID 4
RAID 4 breaks up host data into chunks, calculates parity by performing an
exclusive-or on the chunks, and then writes the chunks to all but one drive in
the array and the parity data to the last drive. When the host requests data from
the disk drives, the controller retrieves the chunks containing the addressed
data, reconstitutes the data from the chunks, and passes the data to the host.
Figure 2-7. Diagram of a RAID 4 write
In the event of a single drive failure, a RAID 4 array will continue to operate
in degraded mode. If the failed drive is a data drive, writes will continue as
normal, except no data will be written to the failed drive. Reads will
reconstruct the data on the failed drive by performing an exclusive-or
operation on the remaining data in the stripe and the parity for that stripe. If
the failed drive is a parity drive, writes will occur, as normal except no parity
will be written. Reads will simply retrieve data from the data disks. There will
be no deterioration in controller performance while a RAID set is in degraded
mode.