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RAID Striping 0 to 4

RAID 0 Striping

With RAID 0 the data is written across multiple drives to improve access performance. There is no data redundancy. So, a 4Meg file would be written across 4 drives in 1Meg pieces. Because of this, the failure of one drive will render the data inaccessible, but the advantage is considerably higher throughput.

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RAID 1 (mirroring without parity or striping)

With RAID 1 data is written identically to two drives, thereby producing a ‘mirrored set’. The read request is examined by either of the two HDDs containing the requested data, whichever one involves the least seek time plus rotational latency.

Similarly, a written request updates the stripes of both drives. The write performance depends on the slower of the two writes, such as, the one that involves a larger seek time and rotational latency.

At least two drives are required to constitute such an array. While more constituent drives may be employed, but many applications deal with a maximum of only two. The array continues to operate if at least one drive is functioning.

Key: A, B, C, D, E, and F are blocks of data

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RAID 2 has bit-level striping with dedicated Hamming-code parity, all disk spindle rotation is synchronized, and data is striped so that each sequential bit is on a different drive. Hamming-code parity is calculated across matching bits and stored on at least one parity drive.

RAID 3 has byte-level striping with dedicated parity, all disk spindle rotation is synchronized and data are striped so each sequential byte is on a different drive. Parity is calculated across matching bytes and stored on a dedicated parity drive.

RAID 4 is block-level striping with dedicated parity and is equivalent to RAID 5 but all parity data is stored on a single drive. In this configuration files can be dispersed between multiple drives. Each drive operates independently allowing I/O requests to be performed in parallel.

RAID 2 is more of a theoretical system and not used in practice. But although applications for RAID 3 exist it is not commonly used in practice.