Two major challenges facing the storage industry today are keeping pace with the increasing performance demands of computer systems by improving disk I/O throughput and providing data accessibility in the face of hard disk failures.
The idea of RAID (Redundant Array of Independent Disks) was first introduced by David A. Patterson, Garth Gibson and Randy H. Katz at the University California at Berkeley in 1988. RAID is a purpose of storing the same data in different places on multiple hard disks and improves storage subsystem performance. The advantage of RAID is to provide better throughput performance and/or data fault tolerance. Better performance is accomplished by sharing the workload in parallel among multiple physical hard drives. Fault-tolerance is achieved through data redundant operation where if one (or more) drive fails or has a sector failure, a mirrored copy of the data can be found on another drive(s).
A RAID appears to the operating system to be a single logical hard disk. The RAID controller controls how the data is stored and accessed across the physical and logical arrays. The RAID controller help users to ensure that the operating system only sees the logical drives and users do not need to worry about managing the complicated schema.
For optimal performance results, select identical hard drives to install in disk arrays. The drives� matched performance allows the array to function better as a single drive.
What are the RAID levels?
Striping / Span (RAID 0)
RAID level 0, which is the fastest drive array you can have, is a performance-oriented disk mapping method. The data in this array gets written across a stripe or different disks for a faster transfer. This technique has striping but no redundancy of data. It offers the best performance but no fault-tolerance. Reads and writes sector of data interleaved between multiple drives. When any disk member fails, it affects the entire array. Performance is better than a single drive since the workload is balanced between the array members. This array type is for high performance systems. Identical drives are recommended for performance as well as data storage efficiency. The disk array data capacity is equal to the number of members times the smallest member capacity. For example, one 40GB and one 60GB drives will form an 80GB (40GBx2) disk array.
Mirroring (RAID 1)
RAID level 1 uses at least two duplicate hard drives and store the exact same blocks of information between them. This is the slowest form of fault tolerance because the data has to be replicated onto two disks at the same time. However, this is the simplest way to provide high reliability.
If one of the mirrored drives suffers a mechanical failure or does not respond, the remaining drive will continue to serve and provide correct data. If one drive has a physical sector error, the mirrored drive will continue to function.
Due to redundancy, the drive capacity of the array is half the total drive capacity. For example, two 40GB drives that have a combined capacity of 80GB would have 40GB of usable storage. With drives of different capacities, there may be unused capacity on the larger drive. RAID 1 increases cost as it takes twice as much hard drives to build our arrays.
Striping with Mirroring (RAID 0+1)
As the name would suggest, RAID 0+1 is striping and mirroring combined. This RAID combines the best of both RAID 0 and RAID 1. It takes a Disk stripe using two disks, and mirrors it to another set of disks for fault tolerance. Data is stripped across several disks, each disk has partner with exactly the same data on it. You get the benefits of fast data access as in RAID 0, with the fault tolerance of RAID 1. This configuration provides optimal speed and reliability. You need double the number of disks as a RAID 0, half for each side of the mirror. At least 4 hard disks are needed while performing RAID 0+1. There are other RAID configurations in addition to those described here, but these are the types most commonly used in the industry.
HDD Capacity of RAID Levels
