vRAID

RAID (Redundant Array of Independent Disks) is a storage virtualization technology, developed in the late 1980s, which allows a large number of inexpensive hard drives to be linked together to form a single high capacity storage device that offers superior performance, storage capacity, and reliability over previous storage solutions. RAID is a form of parallel I/O processing that spreads the workload over a number of disk devices, summing their performance, while providing four primary advantages over stand-alone disks. Those advantages are redundancy, higher performance, increased fault tolerance, and lower costs. Physical RAID volumes are presented as a single, logical hard drive to the Operating System (OS). RAID utilizes a method known as disk striping to partition the storage space on each drive into logical units that can range from a single sector of 512 bytes up to several megabytes. These stripes on the disks of an array are interleaved and addressed in order. The various striping methods are referred to as RAID levels.

A RAID Rebuild is a process by which a RAID array is reconstructed in the event of a hard drive failure. The RAID array copies data to a spare drive while the failed drive is replaced. The RAID array then reconstructs data on the new drive using special RAID algorithms and parity data. RAID rebuild times are a serious concern for storage administrators since during the rebuild process, the performance of applications and processes are impacted by latency, which in turn impacts users’ productivity. High-density drives or multiple drive failures can result in extended rebuild times that can take days or even longer.

Parity, or more properly, the parity bit is a simple form of error detection that checks to see if data is lost or overwritten when moving it from one place in storage to another or when transmitting it between computers. Since data transmission in not entirely an error-free process, data may not be received the same way it was transmitted. The parity bit adds checksums into the data that enable the target device to determine if the data was received correctly.

1. There are at least nine types of RAID plus a non-redundant array (RAID-0) with the most commonly used RAID levels today being 5, 6 and 60 as described below:

• RAID-5: This standard type includes a rotating parity array, thus addressing the write limitation in RAID-4. Consequently, all read and write operations can be overlapped. RAID-5 stores parity information but not redundant data (although parity information can be used to reconstruct data). RAID-5 requires at least three and usually five disks for the array. It's best for multi-user systems in which performance is not critical or which do few write operations.
• RAID-6: This standard type is similar to RAID-5 but includes a second parity scheme that is distributed across different drives and thus offers extremely high fault and drive-failure tolerance.
• RAID-60: This is a type of Nested (Hybrid) RAID level that combines the block-level striping feature RAID level 0 with the distributed dual parity of RAID level 6. It contains the same multi-level disk set as RAID 6 but supports more drives. It requires a minimum of 8 disks in order to operate.