What is the difference between RAID and Mirroring?

This is a common question.

In computing, the acronym RAID (originally redundant array of inexpensive disks, now also known as redundant array of independent disks) refers to a data storage scheme using multiple hard drives to share or replicate data among the drives. Depending on the configuration of the RAID (typically referred to as the RAID level), the benefit of RAID is one or more of increased data integrity, fault-tolerance, throughput or capacity compared to single drives.

When computer people talk about RAID, they generally refer to RAID-5. RAID-5 includes a rotating parity array. (This means if there are 4 disks in an array, data is written to 3 of the disk units and space on the 4^th drive is used for parity – or a way to validate the data so that if a drive in the array fails, the data can be reconstructed on the remaining 3 drives.) Thus, all read and write operations can be overlapped. RAID-5 stores parity information but not redundant data (but 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 5

As you can see in the above diagram, data is represented by A1, A2, and A3. Ap is the parity information. Notice below “A” is data as B1, B2 and B3, and Bp is the parity information distributed on a different drive. The concept is to distribute the parity data equally on the drives as well as the data.

Mirroring is another form of RAID – RAID-1 for the purist. Mirroring consists of at least 2 disk drives that duplicate the storage of data. More frequently, you will see 2 or disk units on each array so duplicate data is sent to the second array of disks. As such, if 1 disk drive fails in the first array, the system fails over to the second array of functional drives so the system can continue to operate. This gives you continuous operation while you wait to have the failed drive repaired and re-instate Mirroring.

As you can see in this diagram, A1 data on Array 1 has a duplicate copy of A1 on Array2, and so forth for A2, A3, and A4.