Redundant Array of Independent Disks (RAID) combines multiple physical disks into one logical storage unit. The acronym RAID originally stood for Redundant Array of Inexpensive Disks. RAID can provide improved performance and fault tolerance over single-disk storage. RAID is defined over several distinct levels, each of which has different requirements and features.
RAID 0 spreads data over multiple stripes on multiple disks.
RAID 1 mirrors all data from one disk onto a second disk, providing a complete copy.
RAID 2 (obsolete) spreads data at the bit level over multiple stripes on multiple disks. Error correction is provided through Hamming-code parity.
RAID 3 spreads data at the byte level over multiple stripes on multiple disks. There is a dedicated parity disk.
RAID 4 spreads data at the block level over multiple stripes on multiple disks. There is a dedicated parity disk.
RAID 5 spreads data at the block level over multiple stripes on multiple disks. Parity stripes are spread over multiple disks.
RAID 6 spreads data at the block level over multiple stripes on multiple disks. Parity stripes are spread over multiple disks, and parity stripes are doubled over RAID 5.
Higher levels of RAID have been defined by various vendors, but have not been standardized across the industry.
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