THEORY - 101:- INTRODUCTION TO RAID

Introduction : RAID 

RAID is the organization of multiple disks into a large, high performance logical disk.

Disk arrays stripe data across multiple disks and access them in parallel to achieve:

• Higher data transfer rates on large data accesses and
• Higher I/O rates on small data access

Large disk arrays, however are highly vulnerable to disk failures. A disk array with a hundred disks is a hundred times more likely to fail than a single disk. An MTTF (mean‐time‐to‐failure) 500,000 hours for a single disk implies an MTTF of 500,000/100 i.e. 5000 hours for a disk array with a hundred disks. 

The solution to the problem of lower reliability in disk arrays is to improve the availability of the system. This can be achieved by employing redundancy in the form of error‐correcting codes to tolerate disk failures. A redundant disk array can now retain data for much longer time than an unprotected single disk.

Do not confuse between reliability and availability.

Reliability is how well a system can work without any failures in its components. If there is a failure, the system was not reliable.

Availability is how well a system can work in times of a failure. If a system is able to work even in the presence of a failure of one or more system components, the system is said to be available.

 

Redundancy improves the availability of a system, but cannot improve the reliability. Reliability can only be increased by improving manufacturing technologies or using lesser individual components in a system. 

Need of RAID:

• An array of multiple disks accessed in parallel will give greater throughput than a single disk.
• Redundant data on multiple disks provides fault tolerance.
• Performance improvement over a single disk.

With a single hard disk, you cannot protect yourself against the costs of a disk failure, the time required to obtain and install a replacement disk, reinstall the operating system, restore files from backup tapes, and repeat all the data entry performed since the last backup was made.

With multiple disks and a suitable redundancy scheme, your system can stay up and running when a disk fails, and even while the replacement disk is being installed and its data restored.

 

To create an optimal cost‐effective RAID configuration, we need to simultaneously achieve the following goals:

• Maximize the number of disks being accessed in parallel.
• Minimize the amount of disk space being used for redundant data.
• Minimize the overhead required to achieve the above goals.

RAID LEVEL:

Standard RIAD level  

RAID 0 :‐ Stripping

Use for non‐critical storage of data that have to be read/written at a high speed

RAID 1:‐ Mirroring

 Use for mission critical storage, for instance for accounting system. It is suitable for small server in which only two disk will be used.

RAID 3                 

RAID 3 provide high throughput for large data transfer  

RAID 5

RAID 5 is a good all round system that combine efficient storage with excellent security and decent performance.