RAID
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.