6.3. RAID
What is RAID?
Redundant Arrays of Independent Disks (RAIDs) are commonly used. This array is a group of physical disks that uses a certain common method to distribute data across the disks. The data is distributed by stripe units. A stripe unit is the portion of data that is written to one disk drive immediately before the write operation continues on the next drive. When the last drive in the array is reached, the write operation continues on the first drive in the block that is adjacent to the previous stripe unit written to this drive, and so on.
Stripe
The group of stripe units that is subsequently written to all drives in the array (from the first drive to the last) before the write operation continues on the first drive is called a stripe, and the process of distributing data is called striping.
Stripe unit
A stripe unit is a minimal element that can be read from or written to the RAID array, and stripe units contain data or recovery information.
RAID level
The particular striping method that is used for data distribution is also known as the RAID level. This level has certain levels of availability, performance, and available storage capacity because achieving redundancy always lessens disk space that is reserved for storing recovery information. There are basic RAID levels (0, 1, 2, 3, 4, 5, and 6) and spanned RAID levels (00, 10, 50, 60, etc.).
Spanned RAID arrays
Spanned RAID arrays combine two or more basic RAID arrays to provide higher performance, capacity, and availability by overcoming the limitation of the maximum number of drives per array that is supported by a particular RAID controller.
Features of RAIDs
- Increased performance Striping data across multiple disks allows to access them in parallel. This allows: - higher data transfer rates when accessing large volumes of data - higher I/O rates when accessing little chunks of data.
- Increased availability Availability is a measure of how a system is available when one or more of its components fails. When the system works despite the failure of its components, the system is said to be available.
Redundancy
Large number of disks used for striping decreases the reliability of the system. It is therefore important to introduce some redundancy into the array, so disc failures do not affect the operation of the array and do not cause data to be lost. This can be achieved by either introducing mirroring, or by using an additional disc for parity. Parity data can either be stored on a separate disc, or be distributed uniformly across all the discs. The distributed option is more efficient as it distributes the parity operations across the system.