RAID Array - What Is It And What Types Are There ?

RAID stands for "Redundant Array of Inexpensive Disks"
Marketing and advertising firms representing industry RAID manufacturers have been attempting to change the accepted acronym to stand for "redundant array of independent disks" as a means of dis-associating a "low cost" expectation from the RAID technology. The truth is that RAID can be cheap and is an easy way of expanding the storage capacity of your home PC. If you have multiple drives for storage on your PC it is likely that you are using a  type of RAID at the moment. 

This article explains the most common types of RAID and will give you an insight into common use scenarios and terminology.

"RAID" is now used as a general term for data storage schemes that can divide and/or replicate data among multiple re-writeable storage devices. It is most commonly used when referring to hard drives.

There are different types of RAID configurations, the major ones will be listed below. RAID arrays are usually used as a simple means to expand storage requirements, increase the read and write speeds of stored data (input/output performance) and/or to protect data from loss.

To the operating system a RAID appears to be a single logical hard disk.
Quick definitions to make it easier to understand the rest of the article:

Redundant / Data redundancy :
It is a property of some disk arrays which provides fault tolerance. It allows all or part of the data stored to be recovered in the case of disk failure or some other catastrophic event. Redundancy is commonly achieved by duplicating the data elsewhere or providing a means of reconstructing the data reliably.
Because redundancy requires either duplication of the data or error-correcting code to be stored, it usually increases storage requirements.

Parity and Parity Bits:
Parity is a technique of checking whether data has been lost or written over when it's moved from one place in storage to another or even when transmitted between computers. It means "equivalent" or "same" in Latin so it is a simple method that checks to see if one piece of data is the same as it was previously or if it is the same as the data transferred. 
In computers you may hear about a "parity bit", this is a bit of data (as in bits, bytes and megabytes) added to data that allows the computer to check if the data is correct or equivalent to what should be stored. Basically the number of bits in the data is counted and if the total number of data bits is even, the parity bit is set to one so that the total number of bits transmitted will form an odd number (the even value plus the parity bit). If the total number of data bits is already an odd number, the parity bit remains or is set to 0.

Types Of RAID

Concatenation or spanning, often referred to as JBOD ("just a bunch of disks"):
It is a popular method for combining multiple physical disk drives into a single virtual disk. As the name implies, disks are merely concatenated together, so they appear to be a single large disk (added together end to end). Concatenation uses two or more physical drives to create one logical drive. Concatenation is sometimes used to turn several odd-sized drives into one larger useful drive. It provides no data redundancy and you can use drives of different sizes and models.

RAID 0 :
This is a favourite of gamers and users who want to squeeze a lot of performance out of their hardware. Often the read write speeds of hard drives are a bottle neck for computer processing. A RAID 0 can reduce this.
RAID 0 sometimes known as a stripe set or striped volume, splits data evenly across two or more disks (striped) with no parity information for redundancy.
RAID 0 is normally used to increase performance, how is performance increased ?
"If this sentence is data and it is written to disks in RAID 0, and each disk has a read/write speed of 300mb/second it may be stored like this."

When reading the sentence back from the hard drives, instead of having to read at the speed of 300 mb/second (the speed of one of the hard drives), the computer can read from both disks simultaneously. The data is "spanned" across the 2 disks. In the above example the data can be read at 600 mb/second.
A RAID 0 can be created with disks of differing sizes, but the storage space added to the array by each disk is limited to the size of the smallest disk. If a 120 GB disk is added together with a 100 GB disk, the size of the resulting array will be only 200 GB. If you want to implement RAID 0 use drives of the same size and model for the best results.
RAID 0 offers no data redundancy and if one disk fails the data of both disks is lost.

RAID 1 sometimes known as a mirror array :
This creates an exact copy (or mirror) of the data. It needs two or more disks. If you have 2 disks of 100Gb each and set them up as RAID 1 your resulting array will only be 100Gb instead of the 200 Gb total (100Gb is used to "mirror the data") It has a form of 1:1 ratio, a real-time backup. This is useful when read performance or reliability are more important than data storage capacity.


RAID 2, RAID 3 and RAID 4 :
These are uncommon and not generally used privately and have only minor use/implementation commercially.
RAID 2 is obsolete and is therefore not supported commercially. Level 2 stripes data at the bit level rather than the block level. It has Error-Correcting Coding that modern drives do not require , hence it being obsolete.
RAID 3, this is also obsolete, there are however a few manufacturers who still support it. It provides byte-level striping with a dedicated parity disk. RAID 3 cannot service multiple simultaneous read/write requests, also is rarely used.
RAID 4 uses striping (like RAID 0) but with an additional parity disk. If a data disk fails, the parity data is used to create a replacement disk. A disadvantage to Level 4 RAID is that the parity disk can create write bottlenecks / slowdowns.

RAID 5 :
RAID 5 uses striping with parity data distributed across all of the disks. RAID 5 has achieved popularity because of its low cost of redundancy. This can be seen by comparing the number of drives needed to achieve a given capacity. As an example, four 1TB drives can be made into a 2 TB redundant array under RAID 1 or RAID 1+0 (a combination scheme of RAID 1 and RAID 0), but the same four drives can be used to build a 3 TB array under RAID 5. RAID 5 requires all drives except one to be present to in order to continue operation. A minimum of three disks is required for a complete RAID 5 array. RAID 5 implementations can suffer from poor performance when faced with many write cycles that are smaller than the capacity of a single stripe. This is because parity must be updated on each write, this requires a read-modify-write sequences for both the data block and the parity block.

 RAID 6 :
This is a striped set with dual distributed parity. It provides fault tolerance from two drive failures, the array continues to operate with up to two failed drives.
According to the Storage Networking Industry Association (SNIA), the definition of RAID 6 is: "Any form of RAID that can continue to execute read and write requests to all of a RAID array's virtual disks in the presence of any two concurrent disk failures. Several methods, including dual check data computations (parity and Reed-Solomon), orthogonal dual parity check data and diagonal parity, have been used to implement RAID Level 6."

 RAID 6 extends RAID 5 by adding an additional parity block - extra bits to check the integrity of the data
This RAID has striping with parity data distributed across all of the disks. It combines four or more disks in a way that protects data against loss of any two disks. RAID 6 does not have a performance penalty for read operations, but it does have a performance penalty on write operations because of the overhead associated with parity calculations.
 

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