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Page 14 of 15 RAID - Redundant Array of Inexpensive / Independent DisksRedundant Array of Inexpensive Disks, a technology that allowed computer users to achieve high levels of storage reliability from low-cost and less reliable PC-class disk-drive components, via the technique of arranging the devices into arrays for redundancy. More recently, marketers representing RAID manufacturers have revised the term to Redundant Array of Independent Disks, a convenient means of avoiding the expectation of low cost associated with "inexpensive".
"RAID" is now used as an umbrella term for computer data storage schemes that can divide and replicate data among multiple hard disk drives. Various RAID designs involve two key design goals: - Increased data reliability
- Increased input/output performance
There are three key concepts in RAID: - mirroring, the copying of data to more than one disk;
- striping, the splitting of data across more than one disk;
- error correction, where redundant data is stored to allow problems to be detected and possibly fixed (known as fault tolerance).
Different RAID levels use one or more of these techniques, depending on the system requirements.
There are various combinations of these approaches giving different trade-offs of protection against data loss, capacity, and speed. RAID levels 0, 1, and 5 are the most commonly found, and cover most requirements. - RAID 0 (striped disks) distributes data across several disks in a way that gives improved speed and no lost capacity, but all data on all disks will be lost if any one disk fails. Although such an array has no actual redundancy, it is customary to call it RAID 0.
- RAID 1 (mirrored settings/disks) duplicates data across every disk in the array, providing full redundancy. Two (or more) disks each store exactly the same data, at the same time, and at all times. Data is not lost as long as one disk survives. Total capacity of the array equals the capacity of the smallest disk in the array. At any given instant, the contents of each disk in the array are identical to that of every other disk in the array.
- RAID 5 (striped disks with parity) combines three or more disks in a way that protects data against loss of any one disk; the storage capacity of the array is reduced by one disk.
- RAID 6 (striped disks with dual parity) (less common) can recover from the loss of two disks.
- RAID 10 (or 1+0, uses both striping and mirroring) "01" or "0+1" is sometimes distinguished from "10" or "1+0": a striped set of mirrored subsets and a mirrored set of striped subsets are both valid, but distinct, configurations.
Important. RAID is not a good/complete alternative to backing up data. Data may become damaged or destroyed without harm to the drive(s) on which they are stored. For example, part of the data may be overwritten by a system malfunction; a file may be damaged or deleted by user error or malice and not noticed for days or weeks; and, of course, the entire array is at risk of physical damage. RAID Implementation alternatives. RAID combines two or more physical hard disks into a single logical unit by using either special hardware or software. Hardware solutions often are designed to present themselves to the attached system as a single hard drive, so that the operating system would be unaware of the technical workings. For example, you might configure a 1TB RAID 5 array using three 500GB hard drives in hardware RAID, the operating system would simply be presented with a "single" 1TB disk. Software solutions are typically implemented in the operating system and would present the RAID drive as a single drive to applications running upon the operating system.
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