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The redundant array of inexpensive disk essay

RAID redundant array of independent disks; originally redundant array of inexpensive disks is a way of storing the same data in different places on multiple hard disks to protect data in the case of a drive failure. However, not all RAID levels provide redundancy. In their 1988 technical report, "A Case for Redundant Arrays of Inexpensive Disks RAID ," the three argued that an array of inexpensive drives could beat the performance of the top disk drives of the time.

By utilizing redundancy, a RAID array could be more reliable than any one disk drive. While this report was the first to put a name to the concept, the use of redundant disks was already being discussed by others. In 1983, Digital Equipment Corp.

While the levels of RAID listed in the 1988 report essentially put names to technologies that were already in use, creating common terminology for the concept helped stimulate the data storage market to develop more RAID array products. According to Katz, the term inexpensive in the acronym was soon replaced with independent by industry vendors due to the implications of low costs. Because the use of multiple disks increases the mean time between failures MTBFstoring data redundantly also increases fault tolerance.

RAID employs the techniques of disk mirroring or disk striping. Mirroring copies identical data onto more than one drive.

Striping partitions each drive's storage space into units ranging from a sector 512 bytes up to several megabytes. The stripes of all the disks are interleaved and addressed in order.

What RAID level(s) do you use in your organization?

Image of a five-tray RAID hard drive In a single-user system where large recordssuch as medical or other scientific images, are stored, the stripes are typically set up to be small perhaps 512 bytes so that a single record spans all the disks and can be accessed quickly by reading all the disks at the same time. In a multiuser system, better performance requires that you establish a stripe wide enough to hold the typical or maximum size record.

Disk mirroring and disk striping can be combined on a RAID array. Using a RAID controller can improve performance and help protect data in case of a crash. In a hardware-based RAID product, a physical controller manages the array. A physical RAID controller can also be part of the motherboard.

  • CD-ROMs can have their metallic substrate or dye layer scratched off; hard disks can suffer any of several mechanical failures Hardware Components 1438 words - 6 pages overwritten, the hard drive is used to store crucial programming and data;
  • In a hardware-based RAID product, a physical controller manages the array.

With software-based RAIDthe controller uses the resources of the hardware system. While it performs the same functions as a hardware-based RAID controller, software-based RAID controllers may not enable as much of a performance boost.

  • However, not all RAID levels provide redundancy;
  • Each server will be connected to a UPS that is capable of keeping the server running for 45 minutes to give ample time for an orderly shutdown in the case of a Fault Tolerance 1436 words - 6 pages Fault Tolerance is described as a design feature that allows a system to continue operating in spite of errors or problems that occur.

However, with firmware, the RAID system is only implemented at the beginning of the boot process. This numbered system allowed them to differentiate the versions and how they used redundancy and spread data across the array. The number of levels has since expanded and has been broken into three categories: This configuration has striping, but no redundancy of data.

RAID (redundant array of independent disks)

It offers the best performance, but no fault tolerance. Also known as disk mirroring, this configuration consists of at least two drives that duplicate the storage of data. There is no striping. Read performance is improved since either disk can be read at the same time. Write performance is the same as for single disk storage. This configuration uses striping across disks, with some disks storing error checking and correcting ECC information. It has no advantage over RAID 3 and is no longer used.

This technique uses striping and dedicates one drive to storing parity information. The embedded ECC information is used to detect errors. Data recovery is accomplished by calculating the exclusive OR XOR of the information recorded on the other drives. For this reason, RAID 3 is best for single-user systems with long record applications. This level uses large stripes, which means you can read records from any single drive.

This level is based on block -level striping with parity. The parity information is striped across each drive, allowing the array to function even if one drive were to fail.

The array's architecture allows read and write operations to span multiple drives. This results in performance that is usually better than that of a single drive, but not as high as that of a RAID 0 array. RAID 5 requires at least three disks, but it is often recommended to use at least five disks for performance reasons. RAID 5 arrays are generally considered to be a poor choice for use on write-intensive systems because of the performance impact associated with writing parity information.

When a disk does fail, it can take a long time to rebuild a RAID 5 array. Performance is usually degraded during the rebuild time, and the array is vulnerable to an additional disk failure until the rebuild is complete. This technique is similar to RAID 5, but includes a second parity the redundant array of inexpensive disk essay that is distributed across the drives in the array.

The use of additional parity allows the array to continue to function even if two disks fail simultaneously. However, this extra protection comes at a cost. Here are some examples of nested RAID levels. This offers higher performance than RAID 3, but at a much higher cost. It includes a real-time embedded OS as a controller, caching via a high-speed bus and other characteristics of a stand-alone computer. It appears to be similar to RAID 5 with some performance enhancements, as well as the enhancements that come from having a high-speed disk cache on the disk array.

This level, provided by the Linux kernel, supports the creation of nested and nonstandard RAID arrays. By putting multiple hard drives together, RAID can improve on the work the redundant array of inexpensive disk essay a single hard drive and, depending on how it is configured, can increase computer speed and reliability after a crash.

With RAID 0, files are split up and distributed across drives that work together on the same file. As such, reads and writes can be performed faster than with a single drive. RAID 5 arrays break data into sections, but also devote another drive to parity. This parity drive can see what is working when one nonparity drive fails, and can figure out what was on that failed drive.

This function allows RAID to provide increased availability.

With mirroring, RAID arrays can have two drives containing the same data, ensuring one will continue to work if the other fails. Although the term inexpensive was removed from the acronym, RAID can still result in lower costs by using lower-priced disks in large numbers. Nested RAID has become popular in spite of its cost because it helps to overcome some of the reliability problems associated with standard RAID levels.

Initially, all the drives in a RAID array are installed at the same time. This makes the drives the same age and subject to the same operating conditions and amount of wear. But when a drive fails, there is a high probability that another drive in the array will also soon fail.

The problem is that the RAID array and the data it contains are left in a vulnerable state until a failed drive is replaced and the new disk is populated with data. Because drives have much greater capacity now than when RAID was first implemented, it takes a lot longer to rebuild failed drives.

Longer rebuild times increase the chance that a second drive will fail before the first drive is rebuilt. Even if a second disk failure does not occur while the failed disk is being replaced, there is a chance the remaining disks may contain bad sectors or unreadable data.

These types of conditions may make it impossible to fully rebuild the array.

Raid: Redundant Array Of Inexpensive Disks

Nested RAID levels address these problems by providing a greater degree of redundancy, greatly decreasing the chances of an array-level failure due to simultaneous disk failures. Alternatives such as erasure coding offer better data protection albeit at a higher priceand have been developed with the intention of addressing the weaknesses of RAID.

As drive capacity increases, so does the chance for error with a RAID array, and capacities are consistently increasing. SSDs have no moving parts and do not fail as often as hard disk drives. Hyperscale computing also removes the need for RAID by using redundant servers instead of redundant drives.