RAID:Redundant Array of Independent Drives

RAID stands for Redundant Array of Independent Drives(or Disks),another name Redundant Array of Inexpensive Drives (or Disks) usually associated with 2 or more drives.
It is a data storage schemes that divides and or replicates data among multiple drives or disks.This provides data reliability and increased IO performance.
RAID allows you store the same data at multiple places in a balanced way to improve performance.It is a category of disk drives that make use of two or more drives in a combination primarily for fault tolerance and performance too.

Suppose your copying the data on two disks that is (Disk Mirroring or RAID level 1), reading data alternately from each disk improves performance and each disk will be less busy. In addition, if one disk fails, other disk have a copy of data.

What you expect from RAID, is totally depend on RAID level you are going to use. it is designed to protect against *disk* failures, and not against *power* failures or *operator* mistakes, or programming bug from RAID software itself.
There are 2 options available for RAID solution,  one can go with either Hardware RAID solution or a Software RAID solution.
RAID Hardware, is a disk controller to which you can cable up the disk drives. RAID Software, is a set of kernel modules coupled together with management utilities that implement RAID in Software and require no additional hardware.
Check here: Advantages and disadvantages of RAID

There are total 10 types of RAID levels:

RAID level 0 Striped Disk Array without Fault Tolerance
RAID level 1 Mirroring and Duplexing
RAID level 2 Error-Correcting Coding
RAID level 3 Bit-Interleaved Parity
RAID level 4 Dedicated Parity Drive
RAID level 5 Block Interleaved Distributed Parity
RAID level 6 Independent Data Disks with Double Parity


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