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NAS System
14 User Manual
Definition of RAID Levels
RAID 0
is typically defined as a group of
striped disk drives without parity or data
redundancy. RAID 0 arrays can be configured
with large stripes for multi-user environments
or small stripes for single-user systems that
access long sequential records. RAID 0 arrays
deliver the best data storage efficiency and
performance of any array type. The
disadvantage is that if one drive in a RAID 0
array fails, the entire array fails.
RAID 1 , also known as disk mirroring, is
simply a pair of disk drives that store
duplicate data but appear to the computer as
a single drive. Although striping is not used
within a single mirrored drive pair, multiple
RAID 1 arrays can be striped together to
create a single large array consisting of pairs
of mirrored drives. All writes must go to both
drives of a mirrored pair so that the
information on the drives is kept identical.
However, each individual drive can perform
simultaneous, independent read operations.
Mirroring thus doubles the read performance
of a single non-mirrored drive and while the
write performance is unchanged. RAID 1 delivers the best performance of any redundant
array type. In addition, there is less performance degradation during drive failure than in
RAID 5 arrays.
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.