Sans Digital Mobileraid MR8X, Detailed User'S Manual

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SAS-to-SATA II RAID Subsystem
17
Installation and Configuration Manual
RAID 3 sector-stripes data across groups of
drives, but one drive in the group is dedicated to
storing parity information. RAID 3 relies on the
embedded ECC in each sector for error detection.
In the case of drive failure, data recovery is
accomplished by calculating the exclusive OR
(XOR) of the information recorded on the
remaining drives. Records typically span all
drives, which optimizes the disk transfer rate.
Because each I/O request accesses every drive
in the array, RAID 3 arrays can satisfy only one
I/O request at a time. RAID 3 delivers the best
performance for single-user, single-tasking
environments with long records. Synchronized-
spindle drives are required for RAID 3 arrays in order to avoid performance degradation
with short records. RAID 5 arrays with small stripes can yield similar performance to RAID 3
arrays.
Under
RAID 5 parity information is distributed
across all the drives. Since there is no dedicated
parity drive, all drives contain data and read
operations can be overlapped on every drive in the
array. Write operations will typically access one
data drive and one parity drive. However, because
different records store their parity on different
drives, write operations can usually be overlapped.
RAID 6 is similar to RAID 5 in that data protection is achieved by writing parity information
to the physical drives in the array. With RAID 6, however, two sets of parity data are used.
These two sets are different, and each set occupies a capacity equivalent to that of one of
the constituent drives. The main advantage of RAID 6 is High data availability – any two
drives can fail without loss of critical data.