APC RAID Subsystem SCSI-SATA II User Manual download pdf (Page 36)

SCSI to SATA II RAID Subsystem

User 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.

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.

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APC RAID Subsystem SCSI-SATA II User Manual Page 36