RAID (Redundant Array of Independent)

RAID (Redundant Array of Independent) & RAID Types

The term RAID was defined as Redundant Array of Independent (or Inexpensive) Disks,. RAID storage uses multiple disks in order to provide fault tolerance, to improve overall performance, and to increase storage capacity in a system

RAID allows us to store the same data redundantly (in multiple paces). RAID disk drives are used frequently on servers but aren't generally necessary for personal computers.

The disks can be combined into the array in different ways known as RAID levels. Each of RAID levels has its some characteristics:

• Fault-tolerance which is the ability to survive of one or several disk failures.
• Performance  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. We calculate the capacity of the particular RAID type and a set of the member disks on a free online: 

http://www.raid-calculator.com

RAID 0 : RAID based on striping technique doesn't provide fault tolerance .

Advantages

·         RAID 0 performance is very good, both in read and write operations.

·         Storage capacity is used, there is no overhead.

·         The technology is easy to implement.

   Disadvantages

·         There is not fault-tolerant. If one drive will fails, all data in the RAID 0 array are lost. It should not be used for mission-critical systems.

RAID 1: We need at least 2 drives for a RAID 1 array. Data are stored twice by writing them to both the data drive (or set of data drives) and a mirror drive (or set of drives). If a drive fails, the controller uses either the data drive or the mirror drive for data recovery and continues operation.

Advantages

·         It is a simple and easy to implement technology

·         Read operations are fast

·         Offers high fault tolerance for configurations having two disk drives

·         In case a drive fails, data do not have to be rebuild, they just have to be copied to the replacement drive.

Disadvantages

·      Usable data storage capacity is only half of the total drive capacity because data is redundant.

·      Higher cost, as RAID level 1 requires double the amount of drives to achieve the desired capacity.

RAID 5 : RAID 5 combines block-level striping with distributed parity among all drives. `             This is most common and secure RAID level.

RAID 5 requires a minimum of 3 drives and it can work with up to a maximum of 16    

drives. This type of array can withstand the failure of a drive without losing data. The     

extra cache memory is used on these controllers to improve the write performance. RAID

level 5 combines storage with security and performance.

Advantages

·         Read data transactions are fast as compared to write data transactions that are somewhat slow due to the calculation of parity.

·         If a drive fails, you still have access to all data, even while the failed drive is being replaced and the storage controller rebuilds the data on the new drive.

Disadvantages

·         It has a complex technology

·         Failed drives have adverse effects on throughput

RAID 6  Striping with Double Parity

RAID Level 6 is similar to RAID 5 with an added advantage of double distributed parity, which provides fault tolerance up to 2 failed drives.

It can take hours or days to rebuild this RAID array. During the rebuilding of RAID 5 if two drives fail, then data can be lost. But, in RAID 6, the RAID array will survive even after the second drive failure, thereby making RAID 6 more secure than RAID

Advantages

·         Data accessibility is high

·         Higher redundancy compared to RAID 5.

·         If two drives fail, you still have access to all data, even while the failed drives are being replaced. So RAID 6 is more secure than RAID 5.

Disadvantages

·         Due to double parity write data transactions are slow.

·         Rebuilding RAID array takes longer time because of its complex structure.

RAID 10 (RAID 1+0) – Mirroring and Striping

  RAID Level 10 is a combination of RAID 0 and RAID 1.

·         That means mirroring and striping are done in one single RAID array. RAID 10 is a nested or hybrid and sometimes identified as RAID 1 + 0. It provides both security and performance by mirroring and striping data into multiple drives. It requires minimum 4 drives to build RAID Level 10 or (RAID 1+ 0) system.

Advantages

·         Combination of mirroring and striping makes it fast and resilient.

·         Mirroring makes RAID 10 secure

Disadvantages

·         It is expensive

·         Limited scalability