Proware SN-3163S3H-QX, User Manual

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SAS / SATA II NAS System
18 User Manual
RAID 3 sector-stripes data across groups of drives, but one drive in the group is
dedicated for 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 1+0 and RAID 0+5 or
RAID 50.