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What is RAID
Posted by Administrator on 15 January 2008 03:45

What is RAID?

RAID (Redundant Array of Independent Disks) is a set of technology standards for teaming disk drives to improve fault tolerance and performance.

RAID Levels

LevelName
0Striping
1Mirroring
2Parallel Access with Specialized Disks
3Synchronous Access with Dedicated Parity Disk
4Independent Access with Dedicated Parity Disk
5Independent Access with Distributed Parity
6Independent Access with Double Parity

Choosing a RAID Level

Each RAID level represents a set of trade-offs between performance, redundancy, and cost.

RAID 0 -- Optimized for Performance

RAID 0 uses striping to write data across multiple drives simultaneously. This means that when you write a 5GB file across 5 drives, 1GB of data is written to each drive. Parallel reading of data from multiple drives can have a significant positive impact on performance.

The trade-off with RAID 0 is that if one of those drives fail, all of your data is lost and you must retore from backup.

RAID 0 is an excellent choice for cache servers, where the actual data being stored is of little value, but performance is very important.

RAID 1 -- Optimized for Redundancy

RAID 1 uses mirroring to write data to multiple drives. This means that when you write a file, the file is actually written to multiple disks. If one of the disks fails, you simply replace it and rebuild the mirror.

The tradeoff with RAID 1 is cost. With RAID 1, you must purchase double the amount of storage space that your data requires.

RAID 5 -- A Good Compromise

RAID 5 stripes data across multiple disks. RAID 5, however, adds a parity check bit to the data. This slightly reduces available disk capacity, but it also means that the RAID array continues to function if a single disk fails. In the event of a disk failure, you simply replace the failed disk and keep going.

The tradeoffs with RAID 5 are a small performance penalty in write operations and a slight decrease in usabable storage space.

RAID 6 -- Better Redundancy

RAID 6 stripes data across multiple disks. RAID 5, however, adds double parity check bits to the data. This slightly reduces available disk capacity, but it also means that the RAID array continues to function if a two disk fails. In the event of two disks failure, you simply replace the failed disks and keep going.

RAID 1+0 -- Optimize for Performance and Redundancy

RAID 1+0 combines the performance of RAID 0 with the redundancy of RAID 1.

To build a RAID 1+0 array, you first build a set of RAID 1 mirrored disks and you then combine these disk sets in a RAID 0 striped array.

A RAID 1+0 array can survive the loss of one disk from each mirrored pair. RAID 1+0 cannot survive the loss of two disks in the same mirrored pair.

For more information, please also refer to http://en.wikipedia.org/wiki/RAID.


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