| Raid more.. The term .RAID and common definitions for operating levels was made popular by a series of papers and articles in the 1980's. The name is descriptive of fault tolerant concepts, such as disk mirroring (a redundant drive) that had been in use for years. It was originally expressed as Redundant Array of Inexpensive Disks. A significant driving factor in early implementations was the high cost of drives. As it evolved, Inexpensive changed to Independent for better market definition.
Note: Accronyms such as .RAID (RAID), used in this paper may be preceded by periods to assist machine translation. The period is not part of the accronym.
.RAID does not prevent drive failures. Its purpose is to prevent a System failure due to a drive failure. The disk drive is the most mechanicaly dependent component in your computer. When it finally dies or starts to show errors, a redundant drive allows you to continue working. Repairs can be scheduled, a costly emergency and lost productivity can be avoided. The cost of a controller and additional drive is miniscule compared to the cost of an idled assembly line, or the hourly cost of idled workers or operators.
.RAID does not replace the need for backups! Backups, especially off-site backups are a separate, integral part of good system planning. While viewed by some as an on-line backup, .RAID is really a productivity tool. It is used to avoid the productivity impact of a drive failure. A 1993 Wall Street Journal article estimated that Five Billion dollars of annual productivity was lost in the USA that year because of computer system down-time. This cost continues to rise. Fault tolerance and redundancy is an increasingly important economic consideration in the computer dependent workplace.
.RAID levels 0, 1, 5 and 0+1 prevail. Levels 2, 3, 4 & 6 are technically defined but are obsolete or not as widely used. Some numbering is marketing derived, ie. a controller offering a choice of 0, 1, 3 or 5 might be advertised as a RAID-9 controller (add all the numbers and they total 9).
While there is general agreement on the definition of basic .RAID levels, many details are left to the vendor. This provides a rich market offering based on features and implementation. From a marketing view, the benefit of a particular .RAID level or feature is statistically derived based on favorable assumptions about system usage and failure modes of the drives. In real life, individual usage and failure experience varies. This can generate significant debate about the relative merits of different implementations.
.RAID can be accomplished with software, hardware or a combination of both. Hardware solutions can be provided by a specialized card installed in the system, or an external box with electronics and drives. External sub-systems are more costly and have more parts to consider. Centari Computers specializes in internal (card) solutions for OEMs and VARS.
Software solutions use resources (CPU cycles and memory) of the host system, CPU utilization can run as high as 100%. Time lost to this service is not available for system or application programs. Benchmarking on an idle system will not reflect performance under heavy load. Systems that are not heavily loaded can use software solutions because of excess CPU and system capacity. Centari Computers offers software and software-hardware hybrids (DataShadow family) for cost sensitive applications.
Higher performance (Intelli-Cache series) boards have a separate processor and memory to relieve the host processor and improve performance by adding disk caching. Centari Computers excells in support for OEMs and VARs needing cost effective, high performance fault tolerant systems.
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