Redundant Array of Independent (or Inexpensive) Disks. Type of disk, often used on servers, where several physical disks are combined into an array for better speed and fault tolerance.

Level 0 - implements data striping where file blocks are written to separate drives. Does not provide fault tolerance, because failure of one drive will result in data loss.

Level 1 - implements data mirroring. Data is duplicated on two drives either through software or hardware. Provides faster read performance than a single drive.

Level 2 - not used in practice. Data is split at bit level at written to multiple drives.

Level 3 - requires at least 3 drives. Data block is striped at byte level across drives and error correction codes (parity info) is recorder on another drive. Provides fault tolerance but slower writing performance.

Level 4 - Similar to Level 3 but provides faster performance because it uses blocks for striping.

Level 5 - Similar to Level 4 but improves performance but also striping parity info across multiple drives.

Level 6 - Similar to Level 5 but also uses second parity scheme for better fault tolerance.

Level 7 - Proprietary RAID design by Storage Computer Corporation. Faster than other levels because it uses multiple levels of cache and asynchronous I/O transfers. In addition multiple RAID levels can be combined to improve performance or reliability.
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