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Old 11-27-2008, 10:43 AM
john1544 john1544 is offline
Join Date: Nov 2008
Posts: 30
Default Raid

Guys i heard a lot about RAID but i dont know much about RAID so will you tell me :
What is RAID ?
Full form of RAID ?
Where we can use RAID technology ?
Kindly reply soon.
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Old 11-27-2008, 02:56 PM
Alexandre Alexandre is offline
Join Date: Oct 2008
Posts: 72

Purpose and basics

Redundancy in a way that extra data is written across the array, which are organized so that the failure of one (sometimes more) disks in the array will not result in loss of data. A failed disk may be replaced by a new one, and the data on it reconstructed from the remaining data and the extra data. A redundant array allows less data to be stored. For instance, a 2-disk RAID 1 array loses half of the total capacity that would have otherwise been available using both disks independently, and a RAID 5 array with several disks loses the capacity of one disk. Other RAID level arrays are arranged so that they are faster to write to and read from than a single disk.

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 full capacity, but all data on all disks will be lost if any one disk fails.

* RAID 1 (mirrored disks) could be described as a backup solution, using two (possibly more) disks that each store the same data so that data is not lost as long as one disk survives. Total capacity of the array is just the capacity of a single disk. The failure of one drive, in the event of a hardware or software malfunction, does not increase the chance of a failure nor decrease the reliability of the remaining drives (second, third, etc).

* 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 (less common) can recover from the loss of two disks.

* RAID 10 (or 1+0) uses both striping and mirroring.

RAID involves significant computation when reading and writing information. With true RAID hardware the controller does all of this computation work. In other cases the operating system or simpler and less expensive controllers require the host computer's processor to do the computing, which reduces the computer's performance on processor-intensive tasks (see "Software RAID" and "Fake RAID" below). Simpler RAID controllers may provide only levels 0 and 1, which require less processing.

RAID systems with redundancy continue working without interruption when one, or sometimes more, disks of the array fail, although they are vulnerable to further failures. When the bad disk is replaced by a new one the array is rebuilt while the system continues to operate normally. Some systems have to be shut down when removing or adding a drive; others support hot swapping, allowing drives to be replaced without powering down. RAID with hot-swap drives is often used in high availability systems, where it is important that the system keeps running as much of the time as possible.

RAID is not a good 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 catastrophes such as theft, flood, and fire.

You can read more here

RAID - Wikipedia, the free encyclopedia
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