Understanding RAID – Redundant Array of Independent Disks

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What is RAID?

RAID is an acronym for Redundant Array of Independent (or Inexpensive) Disks. In fact, RAID is the way of combining several independent and relatively small disks into a single storage of a large size. The disks included into the array are called array members. The disks can be combined into the array in different ways which are known as RAID levels. Each of RAID levels has its own characteristics of:

  • Fault-tolerance which is the ability to survive of one or several disk failures.
  • Performance which shows the change in the read and write speed of the entire array as compared to a single disk.
  • The capacity of the array which is determined by the amount of user data that can be written to the array. The array capacity depends on the RAID level and does not always match the sum of the sizes of the RAID member disks. To calculate the capacity of the particular RAID type and a set of the member disks you can use a free online RAID calculator.
RAID levels
  • RAID 0 – based on striping. This RAID level doesn’t provide fault tolerance but increases the system performance (high read and write speed).
  • RAID 1 – utilizes mirroring technique, increases read speed in some cases, and provides fault tolerance in the loss of no more than one member disk.
  • RAID 0+1 – based on the combination of striping and mirroring techniques. This RAID level inherits RAID 0 performance and RAID 1 fault tolerance.
  • RAID1E – uses both striping and mirroring techniques, can survive a failure of one member disk or any number of nonadjacent disks. There are three subtypes of RAID 1E layout: near, interleaved, and far. More information and diagrams on the RAID 1E page.
  • RAID 5 – utilizes both striping and parity techniques. Provides the read speed improvement as in RAID 0 approximately, survives the loss of one RAID member disk.
  • RAID 5E – a variation of RAID 5 layout the only difference of which is an integrated spare space allowing to rebuild a failed array immediately in case of a disk failure. Read more on the RAID5E page.
  • RAID 5 with delayed parity – pretty similar to basic RAID 5 layout, but uses nonstandard scheme of striping. More information about RAID5 with delayed parity.
  • RAID 6 – similar to RAID 5 but uses two different parity functions. The read speed is the same as in RAID 5.

How RAID is organized?

Two independent aspects are clearly distinguished in the RAID organization.

  1. The organization of data in the array (RAID storage techniques: striping, mirroring, parity, combination of them).
  2. Implementation of each particular RAID installation – hardware or software.
RAID storage techniques

The main methods of storing data in the array are:

  • Striping – splitting the flow of data into blocks of a certain size (called “block size”) then writing of these blocks across the RAID one by one. This way of data storage affects on the performance.
  • Mirroring is a storage technique in which the identical copies of data are stored on the RAID members simultaneously. This type of data placement affects the fault tolerance as well as the performance.
  • Parity is a storage technique which is utilized striping and checksum methods. In parity technique, a certain parity function is calculated for the data blocks. If a drive fails, the missing block are recalculated from the checksum, providing the RAID fault tolerance.

All the existing RAID types are based on striping, mirroring, parity, or combination of these storage techniques.

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