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Redundant array of independent memory
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(Redundant Array of Independent Disks) A disk subsystem that increases performance or provides fault tolerance or both. RAID uses two or more regular hard drives and a RAID controller, which is plugged into motherboards that do not have RAID circuits. Today, most motherboards have built-in RAID but not necessarily every RAID configuration (see below). In the past, RAID has also been accomplished by software only but was much slower. In the late 1980s, the "I" in RAID stood for "inexpensive" but was later changed to "independent."
In large storage area networks (SANs), floor-standing RAID units are common with terabytes of storage and huge amounts of cache memory. RAID is also used in desktop computers by gamers for speed and by business users for reliability. Following are the various RAID configurations. See NAS and SAN.
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RAID 0 - Disk Striping for Performance (Popular)
Widely used for gaming, disk striping interleaves data across multiple drives for performance. However, there are no safeguards against failure. See RAID 0.
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RAID 1 - Mirroring for Fault Tolerance (Popular)
Widely used, RAID 1 writes two drives at the same time. It provides the highest reliability but doubles the number of drives needed.
RAID 10 combines RAID 1 mirroring with RAID 0 striping for both safety and performance. See RAID 1 and RAID 10.
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RAID 3 - Speed and Fault Tolerance
Data are striped across three or more drives for performance, and parity is computed for safety. Similar to RAID 3, RAID 4 uses block level striping but is not as popular. See RAID 3 and RAID parity.
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RAID 5 - Speed and Fault Tolerance (Popular)
Data are striped across three or more drives for performance, and parity is computed for safety. RAID 5 is similar to RAID 3, except that the parity is distributed to all drives. RAID 6 offers more reliability than RAID 5 by performing more parity computations. For more details, see RAID 5.
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Redundant array of independent memory
A redundant array of independent memory (RAIM) [1] is a design feature found in certain computers' main random access memory. RAIM utilizes additional memory modules and striping algorithms to protect against the failure of any particular module and keep the memory system operating continuously. RAIM is similar in concept to a redundant array of independent disks (RAID), which protects against the failure of a disk drive, but in the case of memory it supports several DRAM device chipkills and entire memory channel failures. RAIM is much more robust than parity checking and ECC memory technologies which cannot protect against many varieties of memory failures.
On July 22, 2010, IBM introduced the first high end computer server featuring RAIM, the zEnterprise 196. Each z196 machine contains up to 3 TB (usable) of RAIM-protected main memory. In 2011 the business class model z114 was introduced also supporting RAIM. The formal announcement letter offered some additional information regarding the implementation:
[...] IBM's most robust error correction to date can be found in the memory subsystem. A new redundant array of independent memory (RAIM) technology is being introduced to provide protection at the dynamic random access memory (DRAM), dual inline memory module (DIMM), and memory channel level. Three full DRAM failures per rank can be corrected. DIMM level failures, including components such as the controller application specific integrated circuit (ASIC), the power regulators, the clocks, and the board, can be corrected. Memory channel failures such as signal lines, control lines, and drivers/receivers on the MCM can be corrected. Upstream and downstream data signals can be spared using two spare wires on both the upstream and downstream paths. One of these signals can be used to spare a clock signal line (one upstream and one downstream). Together these improvements are designed to deliver System z's most resilient memory subsystem to date.[2]
See also
References
- ^ Meaney, P. J. (Jan.-Feb. 2012). IBM zEnterprise redundant array of independent memory subsystem .IBM Journal of Research and Development.
- ^ "Formal Announcement Letter for zEnterprise". IBM Corp.. 2010-07-22. http://www-01.ibm.com/cgi-bin/common/ssi/ssialias?infotype=an&subtype=ca&htmlfid=877/ENUSZG10-0249&appname=isource&language=enus#h2-descx.
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