RAID (Redundant Array of Inexpensive Disks)

RAID is a method of combining several hard drives into one unit. It can offer fault tolerance and higher throughput levels than a single hard drive or group of independent hard drives. 

Fundamental to RAID technology is striping. Data in a striped array is interleaved across all the drives in the array. Data is read and written on both drives at the same time. The performance of a striped array is governed by the stripe width and stripe size. The width is equal to the number of drives in your array. The size can be as small as one sector (512 bytes) or as large as several megabytes. The specific type of operating environment determines whether large or small stripes should be used. RAID provides real-time data recovery when a hard drive fails, increasing system uptime and network availability while protecting against loss of data. Multiple drives working together also increase system performance.

The RAID controller adds a parity bit to all binary data being written to an array, then is added up to equal either an even or an odd number. By analyzing this value, the controller can determine whether the information has been compromised in any way. If it has, it can replace the data automatically with data from the other drive. It analyzes bits then returns either a TRUE or FALSE. The controller can fill in the missing information. If you see an equation like 4 + __ = 6, you know the blank is supposed to be a 2. The XOR logic is used in this way to rebuild corrupted data on the array, thus maintaining integrity. Most parity checks use the XOR for fault tolerance. This is a type of Boolean logic (eXclusive OR).

Software-based RAID - Included in network operating systems such as Novell NetWare and Microsoft Windows NT/2000.®  All RAID functions are handled by the host CPU, which can tax its ability to perform other computations by up to 18%.

The following RAID levels are supported by most network operating systems:

Hardware-based SCSI RAID - Processor-intensive RAID operations are offloaded from the host CPU to enhance performance.

A research group at the University of California in Berkeley defined six RAID levels. Understanding these levels is important, because each level is optimized for a different use.

• RAID Level 0 is not fault tolerant. Data is split across drives, resulting in higher data throughput, but the failure of any disk in the array results in a total loss of data.
• RAID Level 1 is commonly referred to as mirroring with 2 hard drives. It provides fault tolerance by duplicating all data from one drive on another drive. If the first drive fails the second drive contains a mirrored copy, thus no data is lost.  
• RAID Level 2 is rarely used since it is intended for use with drives which do not have built-in error detection. All SCSI drives support built-in error detection, so this level is not important.
• RAID Level 3 is similar to level 4 as parity stored on one drive. Data is striped at a byte level across several drives. 
• RAID Level 4 stripes data at a block level across several drives, with parity stored on one drive. The parity information allows recovery from the failure of any single drive. Although the performance of a level 4 array is very good for reads, writes require that parity data be updated each time. This slows small random writes, in particular, though large writes or sequential writes are fairly fast. 
• RAID Level 5 is commonly referred to as striping with distributed parity and is similar to level 4, but distributes parity among the drives. Although speed for small writes in multiprocessing systems tend to be quicker, the performance for reads tends to be considerably lower than a level 4 array. 
• RAID 0/1 or RAID 10 is a dual level array that utilizes multiple RAID1 sets into a single array. Data is striped across all mirrored sets. This level array offers high data transfer advantages of striped arrays and increased data accessibility.
• RAID 0/5 or RAID 50 is a dual level array that utilizes multiple RAID5 sets into a single array. In RAID 0/5 array, a single hard drive failure can occur in each of the RAID5 without any loss of data on the entire array.