RAID 0
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Introduction
One of a number of different RAID levels that combines two or more hard drives in order to increase the bandwidth provided to the system. In these arrays, blocks of data (commonly called 'stripes') are alternated between each drive in the array. Since all of the drives can opperate in parallel, the transfer rates of sequential tasks are multiplied by the number of drives in the array.
Note that the acronym RAID (Redundant Array of Innexpensive Disks) is a misnomer for this type of array. These arrays have no redundancy and actually represent an increase in the risk of data loss. Please see the reliability section for more details.
Method of Operation
When an array is opperating in RAID 0 mode, data being stored on the drive is seperated into discrete blocks. These blocks are then alternated between all of the drives in the array. In a two drive RAID 0 array, for instance, odd blocks will be placed on the first drive and even ones on the second.
As each drive in the array can operate in parallel, this means that more than one block can be stored/retreived at any given time. In the two drive example given above, for instance, disk A can retreive block 1 while disc B is retreiving block 2. On a conventional hard drive, the system would need to wait for block 1 to be retreived before it could begin working on the second block.
As such, when handling a sequential data transfer a two disc RAID 0 array can theoretically double the throughput. A three disc array can theoretically tripple the throughput and so on. This is true for both read and write operations, as the interleaving process benefits both tasks equally. Naturally, throughput is not the only factor in overall hard drive performance so this does not always translate into improvements of this magnitude in the real-world.
Reliability
As mentioned above, the usage of the term RAID to describe this mode is a bit of a misnomer. Unlike most other RAID levels, RAID 0 has no redundancy and actually increases the risk of losing data on the array. Since the data is interleaved between all drives in the array, the failure of any one of them generally means that all of the data stored on the array will be rendered unusable. This means that a two drive array is twice as likely to fail as a conventional hard drive.
As such, RAID 0 stripesets are not really suitable for storage of data that you can't afford to lose. Their primary utility is for tasks where the files stored on the drive can be easilly recreated and high sequential bandwidth is necessary. In these cases the loss of the array is not critical, so the increased risk is justified by the benefits.
Overhead
As RAID 0 arrays do not store any redundant data, there is no overhead involved. As such, the capacity of a RAID 0 array is the same as the total capacity of all of its constituant drives. An array composed of two 160GB hard drives would result in an array with 320GB of capacity. This is one of the reasons why RAID 0 arrays are so popular, as the proliference of on-board RAID controllers means that they are effectively a 'free' way to boost performance.
Requirements
A RAID 0 array requires a minimum of two hard drives of equal size as well as a hard drive controller than supports it. It is generally a good idea for the two drives to be of the same make and model to ensure maximum performance.
See Also
- RAID - General overview of RAID and all of its different levels.
- RAID 1 - Mirrors all data onto all of the drives contained in the array. This provides the highest level of protection, however it is also relatively inefficient.
- RAID 2 - Similar to RAID 0 but stripes at the bit level rather than the block level. This level is very uncommon and not supported by any modern RAID controllers.
- RAID 3 - Uses byte-level striping with a dedicated parity disc. This can provide similar performance to RAID 0 but can survive the failure of one drive.
- RAID 4 - Same as RAID 3, but uses block-level striping.
- RAID 5 - Uses block-level striping with a distributed parity disc. This can provide similar performance to RAID 0 but can survive the failure of one drive.
- RAID 6 - Uses block-level striping with two distributed parity discs. This can provide similar performance to RAID 0 but can survive the failure of up to two drives.
- RAID 10 - A combonation of RAID 0 and RAID 1. Data is striped across two RAID 1 arrays. This provides the performance advantages of RAID 0 and can survive at least one drive failure.
- RAID 0+1 - A combonation of RAID 0 and RAID 1. In this mode, two RAID 0 arrays are mirrored. This provides the performance advantages of RAID 0 and can survive at least one drive failure.
- Matrix RAID - A propreitary technology used by some Intel chipsets that allows drives to be partitioned and different RAID levels used on each block.
