A Shared Storage Glossary

RAID: Redundant Array of Inexpensive Drives

This is a term that covers various ways of linking multiple drives together to improve transfer speeds and, in some configurations, to provide crash-protection. RAID comes in flavors that are known by their numbers: RAID 0 through RAID 5. For video, RAID 0,3, and 5 are the most common (RAID 2 and 4 are no longer in use). Unity MediaNet is based on a unique combination of RAID 0 and RAID 1. Transoft FibreNet offers RAID 0 or RAID 3.

• RAID Level 0 is not redundant, hence does not truly fit the “RAID” acronym. Data is simply split across drives, resulting in higher data throughput. Since no redundant information is stored, performance is very good, but the failure of any disk in the array results in data loss. This level is commonly referred to as striping.

• RAID Level 1 provides redundancy by writing all data to two or more drives. The performance of a level 1 array tends to be faster on reads and slower on writes compared to a single drive, but if either drive fails, no data is lost. This is a good entry-level redundant system, since only two drives are required; however, since one drive is used to store a duplicate of the data, the cost per megabyte is high. This level is commonly referred to as mirroring.

• RAID Level 3 stripes data at a byte level across several drives, with “parity” information stored on one drive in the set. The parity information allows recovery from the failure of any single drive, but it wastes space because an entire drive in each set is used for parity. Performance is very good for reads (the same as level 0). Writes, however, require that parity data be updated each time. This slows small random writes, in particular, though large writes or sequential writes are fairly fast. Byte-level striping requires hardware support for efficient use.

• RAID Level 5 is similar to level 3, but it stripes data at a block level (in multi-byte chunks of data) and distributes parity among the drives. This can speed small writes in multiprocessing systems, since the parity disk does not become a bottleneck. But because parity data must be skipped on each drive during reads, read performance can suffer.

RAID Controller: to create a RAID array, drives are linked together via software or hardware. In a hardware RAID, a physical controller is used. The computer accessing the storage only sees the controller, which deals with the drives directly. Software RAID can be as simple as using software to “stripe” drives together to behave as one. Or, as it can be as complex as using a dedicated server computer as a “traffic cop” to manage data flow to and from a group of drives, as in Unity.

Controller Card: Every computer connected to a shared storage scheme requires a SCSI or Fibre Channel card that connects it to the storage. Vendors include Adaptec, Atto, Emulex, and JNI.

JBOD (pronounced ‘Jay-Bod’): Just a Bunch of Drives

A group of drives sold as a unit, typically enclosed in a case or housing.

SCSI: Small Computer Systems Interconnect

The old faithful of disk connection schemes. It comes in a host of flavors and has gained considerable speed over the years. The latest high-speed version is known as Ultra-320 SCSI, which is just now coming onto the market. The previous high-speed king, Ultra-160, offers very fast transfer speeds at reasonable cost.

Fibre Channel:

An alternative way to interconnect drives. More robust than SCSI and better suited to drive sharing. Based on a “loop” network architecture that uses thin copper or optical cables to connect computers and storage over much longer distances than SCSI. The Fibre Channel protocol allows massive amounts of storage to be connected to many clients simultaneously, without sacrificing bandwidth in higher-end setups.

SAN: Storage Area Network

Describes various ways of connecting storage into a network so it can be shared by many computers.

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