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What is RAID?

RAID Levels    RAID 0    RAID 1    RAID 2    RAID 3    RAID 4    RAID 5

Download RAID FAQ PDF    SCSI FAQ's

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RAID stands for "Redundant Array of Inexpensive Disks". The concept of RAID originated from a paper ("A case for Redundant Arrays of Inexpensive Disks" - 1987) published from the University of California at Berkley, proposing that using multiple small and rather inexpensive disks to replace the use of a single large disk to achieve fault-tolerant data redundancy. Another term "Parity Data" was often mentioned together with RAID. The parity data or disk is used to reconstruct data to a failed drive by comparing the data for the remaining drives in the array.


What are those RAID Levels ?

Originally five RAID configurations, levels 1 through 5, were defined. A newer version, "data striping", or level 0, offers some performance advantages over other RAID levels but no data redundancy, so technically it isn't actually RAID.

The most popular RAID level are 0, 1, 3 and 5, their definition of RAID levels are:

RAID 0


Known as disk striping. It combines disks that are used to improve some performance, but there is no logic to protect/recover data. *Synchronized disks are used.

Pros Cons
Fast data transfer on large blocks of data I/O. No parity check for fault-tolerance.


RAID 1


This is known as mirroring, where data is written to two different disks at the same time, and data can be read from either disk.

Pros Cons
Secure against disk failure. Double the cost of storage.


RAID 2

Disk striping with several disks similar to RAID 0, but a small percentage of those disks were set aside to be "check disk," A special Hamming Error Correction Codes is implemented. Not used because of high performance penalty.


RAID 3


Striping data over several disks. Parity interleaves at byte level and is stored in a dedicated disk. *Synchronized disks are required.

Pros

Cons

Fastest for large-file transfer, lower cost for data security.

If the dedicated parity disk failed, all data integrity is lost.


RAID 4

Similar to RAID 3, striping data over several disks. Parity interleaves at block level rather that byte level and is stored in a dedicated parity disk. Unlike RAID 3, Non-synchronized disks are being used. RAID 4 improves read access but suffers form a write penalty since every write must access the parity disk.


RAID 5

Striping data and parity over several disks with no dedicated disk for parity.

Pros Cons
Higher I/O rate for writing data and since no dedicated parity disks, no data loss for any disk failure Not as fast as RAID 3.

* Synchronized disks - Multiple disk are being used to write a single file. This is a function of the disk controller.


 

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Last modified: 08/11/03