RAID Levels

A technology called RAID minimizes the loss of data due to drive failures. RAID was created by a group of scientists at University of California at Berkeley in 1987. When they designed the first RAID systems, RAID stood for Redundant Array of Inexpensive Disks. “Inexpensive” was later changed to “Independent,” but both terms are used interchangeably. The idea is that using two or more drives simultaneously can be faster and more failsafe than using a single, larger drive. Using two drives in parallel (one is a copy of the other) is much safer than a single drive. RAID technology offers both solutions at varying levels of coverage. No single RAID level is superior to any other. The RAID level you implement will depend on your needs and budget.

Level 0 – Striping
RAID level 0 is not fault tolerant because no information is duplicated on the disk system. However, performance is enhanced because the data is striped in 64k strips across all the drives in the array. For example, suppose you had three drives in your array. When the controller needs to write a block of data 256k in size, it writes the first 64k block to disk 1, the next 64k block to disk 2, the next 64k block to disk 3, and the last 64k block to disk 1. Additionally, the total available space is the same as the total physical space. Because the probability of failure is greater for this RAID level, it is not used widely.

Level 1 – Mirroring
RAID level 1 is called mirroring because it maintains an identical twin of the first drive. Since all data that is written to disk 1 (original disk) is also written to disk 2 (shadow disk), disk utilization is only 50 percent. There is also a slight write performance loss due to the overhead of writing to two drives. However, there is a performance gain on disk reads and there is very good fault tolerance. To eliminate the single point of failure here, you can use two controller cards, each responsible for writing to a single disk. This completely duplicates the I/O system and is called duplexing. You should use mirroring if you
• Require a high level of redundancy
• Demand high read performance
• Do not want to deal with the complexity of other RAID systems

Level 2
Level 2 RAID uses striping (like RAID 0), but it is performed at the bit level. This method of writing data to the disk requires a good deal of CPU processing and is inefficient. It is thus not adopted and there are no commercially available RAID 2 solutions.

Level 3
Level 3 RAID uses byte striping similar to level 0 but includes a drive dedicated for parity. The parity can be thought of as the sum of all the bytes contained in the stripes of the other disks. The parity disk provides true fault tolerance because if any of the other drives fail, the information on it can be calculated by subtracting the bytes in the other stripes from the parity stripe. The capacity of RAID level 3 arrays can be expressed as a percentage: n-1/n where n is the total number of drives. Since all disks are involved in every read or write (to calculate the parity stripe), RAID level 3 arrays can process only one transaction at a time. They are not suited for random access of many small data blocks. These systems are best suited for large sequential requests, but it’s pretty rare that you’ll ever see this.

Level 4
Level 4 RAID uses striping at the block level to improve efficiency. Otherwise it dedicates a drive to the parity information like level 3 RAID, and thus has the same capacity of RAID level 3 systems.

Level 5
A popular RAID configuration is level 5 RAID. Here the parity is stored in 64k blocks and placed evenly on all drives. Since the controller can process multiple writes in parallel across the array, RAID level 5 can outperform levels 3 and 4. It doesn’t perform as well as level 0 or 1, however, because of the computation used to calculate the parity information. The capacity of RAID level 5 is the same as levels 3 and 4 and should be considered when redundancy is required but you do not have the money for a mirrored solution or if you require the overall larger capacity afforded by arrays as opposed to single, mirrored drives.

Advanced Levels
RAID level 10 is a striped array that is mirrored. This configuration provides excellent fault tolerance and a performance boost. Any single drive in each array or an entire array can fail without data loss. The capacity is reduced to 1-n/2n for every n drives. RAID level 55 requires three RAID 5 arrays that are combined into one larger RAID 5 array. Just replace each drive in RAID 5 with an array of drives each with its own controller, and you have massive capacity, excellent performance, and nearly perfect redundancy. The ratio of total usable space is the same as in RAID 5 systems (1-n/n).

Source of Information : Microsoft Virtualization with Hyper V


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