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RAID (Redundant Arrays of Inexpensive Disks)
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
RAID defines techniques for combining disk drives into arrays. Data is written across all drives, which improves performance and protects data. The alternative is to use one large drive, which does not have the performance benefits of an array and is a single point of failure.
A RAID appears as a single drive. Data is written evenly across the drives by using a technique called striping. Striping divides data over two or more drives, as shown by the crude example in Figure R-2. The figure shows characters for clarity, but data is usually written in blocks or sectors on each drive. A data file that might take 4 seconds to write on a single drive can be striped to four separate drives in 1 second. Likewise, disk reads are improved because there is a speed advantage in simultaneously reading data from four separate drives.
One form of RAID (level 3, as discussed in the following list) provides redundancy that protects against the failure of one disk in the array. Parity information is generated from the data written to each of the RAID drives, and that parity information is written to a backup drive. If one drive in the array fails, the parity information can be used to rebuild the information that is not available due to the failed drive. However, this parity technique does not provide protection if multiple drives fail. Therefore, some vendors have come up with their own redundancy schemes. Some examples can be found at the Advanced Computer & Network Corp. Web site listed on the related entries page.
Most RAID systems allow hot replacement of disks, which means that disks can be replaced while the system is running. When a disk is replaced, the parity information is used to rebuild the data on the disk. Rebuilding occurs while the operating system continues handling other operations, so there is some loss of performance during the rebuilding operation.
RAID levels are outlined in the following list. As mentioned, other levels of RAID have been developed, but some are proprietary. Additional information can be found at the Web sites listed on the related entries page. Note that RAID levels 1, 3, and 5 are most common, while RAID levels 2, 4, and 6 are rarely implemented in commercial products.
In the event of a failed disk, the most basic systems will require that the entire array go offline until the data on the replacement disk is rebuilt. As mentioned, some systems support hot replacement so that a disk can be replaced while the system continues to run. Data is dynamically reconstructed while the system remains online.
The Adaptec Array Guide provides everything you need to know to understand and build storage array systems. The Web site is listed shortly.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.