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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.
Dual-level RAID
achieves a balance between the increased data availability inherent in
RAID 1 and RAID 5 and the increased read performance inherent in disk striping (RAID 0).
These arrays are sometimes referred to as RAID 0+1 or RAID 10 and RAID 0+5 or RAID 50.