"RAID" stands for Redundant Array of Independent Disks. RAID combine multiple hard drives to oncrease system reliability by protecting against data loss and/or improving performance. RAID can be implemented in software, dependent on the OS & using the system CPU as a controller, or in hardware with an independent controller chip.

Common RAID Levels

• RAID Level 0 ("Striping"): Improves I/O performance by spreading I/O load across two or more drives. Data is broken down into blocks and each block is written to a separate disk drive. Net storage is 100% of net total drive capacity. Requires two drives minimum. (Note: not fault-tolerant/redundant).
• RAID Level 1 ("Mirroring"): The same data is written to two drives for 100% redundancy. Net storage is 50% of net total drive capacity. Requires two drives minimum.
• RAID Level 0+1: RAID Level 1 mirrored array whose segments are RAID 0 arrays. High performance and high fault tolerance. Because all data is duplicated ("mirrored"), twice the storage capacity is required. Net storage is 50% of net total drive capacity. Requires four drives minimum.
• RAID Level 5 ("Striping with Parity"): Combines the speed of striping with the reliability of redundancy. Data is striped across two or more drives, while parity information is written in a constantly changing pattern across the array. If a hard drive fails, the data lost from this drive can be recovered through parity calculations. Net storage is 95% of net total drive capacity, less one drive. Requires three drives minimum.

Common RAID Types

• IDE (Parallel ATA): IDE drives are by far the most common. They use standard IDC 40-pin connectors and 40-conductor or 80-conductor ribbon cables. A compact version of this is used with 2.5" IDE drives, using high-density 44-pin cables.
• Serial ATA (SATA): Serial ATA drives feature a smaller 7-pin data connector and thinner, more flexible cable. SATA drives have lower power consumption and higher speeds than IDE (currently 150MB/s versus 133MB/s or less).
• SCSI: Generally used in high-end storage applications due to its faster speed (up to 320MB/s), reliability, and durability. SCSI is a more expensive option, compared to IDE.