RAID disk arrays are redundant arrays of independent or inexpensive disks (RAID) that share one or more RAID controllers and function as a single, large and fast disk drive. They can be interfaced to bus-based or external RAID controllers, computer boards and standalone modules that use special protocols to control RAID systems. RAID disk arrays differ in terms of number of disks, maximum capacity, maximum data transfer rate, and operating temperature. Some RAID disk arrays are hot-swappable or include indicators such as lights, buzzers, or email notification. Others include redundant fans for system cooling or redundant temperature sensors that measure and/or control the temperature of the array. In many systems, light emitting diodes (LED) are used to show hard disk read/write status. RAID disk arrays can be attached to a chassis or mounted on a panel or rack. Integrally mounted products are soldered, hard-wired, or otherwise permanently attached to the computer hardware, usually a chassis, which houses the RAID system. Free standing devices are commonly available.
RAID disk arrays support many different RAID levels. RAID 0 or “striping” is non-redundant and splits data across hard drives. The failure of any disk in the array results in complete data loss. RAID 1, known as “mirroring with two hard drives”, provides redundancy by duplicating all data from one drive to another. RAID 2 uses hamming codes and is designed for drives that do not have built-in error detection. RAID 3 stripes data at the byte level across several drives, with parity stored on one drive. By contrast, RAID Level 4 strips data at the block level. The parity information allows recovery from the failure of any single drive. RAID 5 is similar to RAID 4, but distributes parity among the drives. In other words, no single disk is dedicated to parity. RAID 10 combines RAID 0 striping and RAID 1 mirroring across multiple drive groups. RAID 30 stripes data across multiple drive groups and encodes individual drives with rotated XOR redundancy. Sometimes referred to as “just a bunch of drives”, JBOD is type of RAID in which each drive is operated independently, like a normal disk controller. RAID disk arrays that use other RAID levels are also available.
RAID disk arrays provide both a disk channel interface and a host interface. Disk channel interfaces use SCSI, ATA, IDE, ATA/IDE variants, or fibre channel. Common ATA/IDE variants include serial ATA (SATA), parallel ATA (PATA), ATA-2, ATA-3, ATA/66, ATA/100 and Ultra-ATA. Fibre channel technology uses optical fibers to connect computers and peripheral devices that require high bandwidth. It functions via a serial data transfer architecture that is compatible with SCSI. The most prominent fibre channel standard is arbitrated loop (FC-AL), which can support full-duplex data transfer rates of 100 MBps. Host interfaces for RAID disk arrays use fibre channel or SCSI as well as universal serial bus (USB), peripheral component interconnect (PCI), and IEEE 1394. Peripheral component interconnect (PCI) is a local bus system designed for high-end computer systems. 32-bit PCI is a 32-bit bus that supports rates of 33 MHz. 64-bit PCI is a 32-bit bus that supports rates of 66 MHz. IEEE 1394, a standard developed by the Institute of Electrical and Electronic Engineers (IEEE), is also known as FireWire®, a registered trademark of Apple Computer, Inc.