RAID (originally redundant array of inexpensive disks; now commonly redundant array of independent disks) is a data storage virtualization technology that combines multiple disk drive components into a logical unit for the purposes of data redundancy and performance improvement.
Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the specific level of redundancy and performance required. The different schemes or architectures are named by the word RAID followed by a number (e.g. RAID 0, RAID 1)Before we start talking about the different RAID types, I'm going to define some basic concepts first.
Fault tolerance defined: Basic fault tolerance in the world of storage means your data is intact even if one or more hard drives fails. Some of the more expensive RAID types permit multiple hard drive failures without loss of data. There are also more advanced forms of fault tolerance in the enterprise storage world called path redundancy (AKA multi-path) which allows different storage controllers and the connectors that connect hard drives to fail without loss in service. Path redundancy isn't considered a RAID technology but it is a form of storage fault tolerance.
Storage performance defined: There are two basic metrics of performance in the world of storage. They are I/O performance and throughput. In general, read performance is more valued than write performance because storage devices spend the majority of their time reading data. I/O (Input/Output) performance is the measure of how many small random read/write requests can be processed in a single second and it is very important in the server world, especially database type applications. IOPS (I/O per second) is the common unit of measurement for I/O performance.
Throughput is the measurement of how much data can be read or written in a single second and it is important in certain server applications and very desirable for home use. Throughput is typically measured in MB/sec (megabytes transferred per second) though mbps (megabits per second) is sometimes also used to describe storage communication speeds. There is sometimes confusion between megabits versus megabytes since they sound alike. For example, 100 megabit FastEthernet might sound faster than a typical hard drive that gets 70 MB/sec but this would be like thinking that 100 ounces weighs more than 70 pounds. In reality, the hard drive is much faster because 70 MB/sec is equivalent to 560 mbps.
RAID techniques defined: There are three fundamental RAID techniques and the various RAID types can use one or more of these techniques. The three fundamental techniques are:
- Mirroring
- Striping
- Striping with parity
