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Abstract

A drive array is a collection of hard disk drives that are grouped together. When we talk about RAID, there is often a distinction between physical drives and arrays and logical drives and arrays. Physical arrays can be divided or grouped together to form one or more logical arrays.

Description of RAID


These logical arrays can be divided into logical drives that the operating system sees. The logical drives are treated as single hard drives and can be partitioned and formatted accordingly.The RAID controller is what manages how the data is stored and accessed across the physical and logical arrays. It ensures that the operating system sees the logical drives only and need not worry about managing the underlying schema.

As far as the system is concerned, it is dealing with regular hard drives. A RAID controller's functions can be implemented in hardware or software. Hardware implementations are better for RAID levels that require large amounts of calculations. With today's incredibly fast processors, software RAID implementations are more feasible, but the CPU still gets bogged-down with large amounts of I/O.

The basic concepts made use of in RAID are:
.Mirroring
.Parity
.ECC
.Exclusive OR
.Striping

Mirroring:

Mirroring involves having two copies of the same data on separate hard drives or drive arrays. So the data is effectively mirrored on another drive. The system writes data simultaneously to both hard drives. This is one of the two data redundancy methods used in RAID to protect from data loss.

The benefit is that when one hard drive or array fails, the system can still continue to operate since there are two copies of data. Downtime is minimal and data recovery is relatively simple. All you need to do is rebuild the data from good copy.