APC iSCSI SATA II - Manual (Page 11)

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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.