Apc iSCSI SATA II User Manual download pdf (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.

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Apc iSCSI SATA II User Manual Page 11