RAID 01
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Introduction
A form of Nested RAID array, RAID 0+1 combines RAID 1 and RAID 0 in order to take advantage of the benefits of both. A RAID 0+1 array is basically a RAID 1 array where each of the component 'discs' are actually RAID 0 arrays. Since all of the data is mirrored across two (or more) stripesets, the failure of any one drive in the array will not cause any loss of data or negative effects on performance. As the two RAID 1 arrays can opperate in parallel, the data striped into them will provide performance boosts similar to what a standard RAID 0 array could have provided. While not as efficient as RAID 5, this type of array does not need to generate parity levels so the processor overhead is significantly lower.
This type of array is especially suited to scenarios where redundancy and speed are both required, but where the computer will be under a heavy computational load. As most RAID 5 controllers use the main processor to generate their parity data, switching to a RAID 0+1 array can leave the computer with more muscle to handle any processor-intensive tasks. Read performance of a RAID 0+1 array will be slightly slower than a RAID 5 array with the same number of discs, however write performance will often be just as good because of the reduction in overhead.
Method of Operation
When operating in RAID 0+1 mode, data being fed to/from the drive is first mirrored as if it were being used on a RAID 1 array. The resulting data is then fed to RAID 0 logic that then stripes those two mirrors onto two (or more) individual discs. For more detail on how these two processes operate, please see the respective articles on those RAID levels.
The result is that, when handling a sequential data transfer, a four disc RAID 0+1 array can theoretically double the throughput. A six 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
While conventional RAID 0 arrays are highly susceptable to drive failures, the mirroring used in this system provides a basic level of protection. As such, these arrays can survive a single drive failure without losing any data or negatively impacting performance. How these systems handle a second drive failure depends on their implementation - some controllers implement both levels in one step, in these cases a second drive failure may be survivable if both were storing the same stripe. Simpler implementations may treat the two stripesets as discrete entities, and these will not be able to survive a second drive failure. As such, controllers using RAID 10 are often better able to deal with the failure of multiple hard drives.
Overhead
A four drive RAID 0+1 array will provide the user with half of the total capacity of drives used to build it. The RAID 0 component of the array does not store any redundant data, however the RAID 1 portion doubles storage requirements so this carries through to RAID 0+1. This efficiency extends to larger RAID 0+1 arrays as well.
Requirements
A RAID 0+1 array requires a minimum of four hard drives of equal size as well as a hard drive controller than supports it. It is generally a good idea for all of the 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 0 - Stripes the data at the block level to maximize performance, however that increases the risk of lost data versus individual disks.
- 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.
- Matrix RAID - A propreitary technology used by some Intel chipsets that allows drives to be partitioned and different RAID levels used on each block.
