RAID

History

• The term RAID was coined by researchers  David Patterson, Garth A. Gibson, and Randy Katz at the University of California, Berkeley in 1987.

Introduction

• RAID stands for ‘Redundant Array of Inexpensive Disks’. Now, it is ‘Redundant Array of Independent Disks’ that signify the performance and reliability gains.

Definition

• RAID is an advanced, specific, physical secondary storage media having increased/unlimited capacity, performance, and reliability. 
• It is mass storage devices having multiple units of the storage media/disks in parallel being used as a single secondary storage device such that its disk performance is improved too much.
• RAID is a data storage device that combines multiple physical disk drive components into one or more logical units using virtualization technology for the purposes of data redundancy, performance improvement etc..  

Features

• In RAID, the disks are organized as an array of disks.
• The basic strategy used in RAID is to replace the large capacity disk drive with multiple smaller capacity disks so that the data on these disks is distributed to allow simultaneous access, thus improving the overall input/output performance.
• It also allows an easy way of incrementing the capacity of the disk.

Levels/Categories of RAID

• Initially, there were five standard levels of RAID, but todays so many variations have evolved.
• RAID levels and their associated data formats are standardized/controlled by an authority called the Storage Networking Industry Association (SNIA) in the Common RAID Disk Drive Format (DDF) standard.
• The levels 2 and 4 are not commercially offered.
• In RAID, Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the required level of redundancy and performance.
• Each level of RAID represents reliability, performance, availability, capacity or mix of these.
• There are following levels of RAID which are given below – 

(A) RAID 0

• • Consists of Striping(data is split between multiple disks)concept i.e. it distributes the contents/data of each file among all the drives in the set, hence the failure of any drive causes the entire RAID 0 volume and all files to be lost i.e. it is not fault-tolerant.
  • RAID 0 is best suited in environments where high-speed read/write speeds are needed such as live streaming video and video editing.
  • No data mirroring and parity concept.
  • Easy-to-implement technology.
  • Complete utilization of storage capacity occurs.
  • Not an ideal choice for sensitive/critical systems.
  • The capacity of a RAID 0 volume is considered as the sum of the capacities of the total drives in the set.

(B) RAID 1

• • Consists of Data Mirroring concept.
  • No Striping and Parity concept.
  • In this RAID, Data is written identically to two or more drives thus ‘mirrored set’ of drives are created. 

(C) RAID 2

• • Consists of Bit-level striping with dedicated Parity value.
  • In this, all disk spindle rotation is synchronized and data is striped such that each sequential bit is on a different drive. The parity is calculated across corresponding bits and stored on at least one parity drive.
  • RAID is now of historical significance only and it was rarely used on some early machines but since 2014 it is not used commercially.

(D) RAID 3

• • Consists of Byte-level striping with dedicated Parity value.
  • Here, all disk spindle rotation is synchronized and data is striped such that each sequential byte is on a different drive. The Parity is calculated across corresponding bytes and stored on a dedicated parity drive.
  • Very high Read/Write data transfer rate.
  • Disk failure has an insignificant impact on throughput.
  • Although its implementations exist, but it is not commonly used in practice.

(E) RAID 4

• • Consists of Block-level striping with dedicated Parity value.
  • The main advantage of RAID 4 over RAID 2 and 3 is I/O parallelism i.e. in RAID 2 and 3, a single read I/O operation requires reading the whole group of data drives, while in RAID 4 one I/O read operation does not have to spread across all data drives.

(F) RAID 5

• • Consists of Block-level striping with distributed parity value.
  • Unlike RAID 4, here parity information is distributed among the drives, requiring all drives but one to be present to operate.
  • Here, upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost.
  • RAID 5 requires at least three disks to operate successfully. 

(G) RAID 6

• • Consists of Block-level striping with double distributed parity value.
  • Here, double parity provides fault tolerance up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems, as large-capacity drives take longer to restore.
  • RAID 6 requires a minimum of four disks to operate successfully.
  • As with RAID 5, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced.

Advantages of RAID

• Provides higher data security.
• The technology is easy to implement.
• Supports high fault tolerance.
• Saves from any possible system crash by regular parity check.
• Extraction of data is easy in this because reading and writing process may be done simultaneously. Thus, availability and performance finally increases.
• All storage capacity is used, there is no overhead.
• Ensures data reliability.

Disadvantages of RAID

• In RAID, roughly half of the storage capacity is used.
• It is comparatively more expensive because needs twice as many drivers.
• It is difficult to replace failed drive.

Use/Applications of RAID

• Mostly used in database storage.
• Can be used as backup device.