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Definition
Information has become a commodity in today's world, and protecting that
information has become mission critical. The Internet has helped push this information
age forward. Popular websites process so much information, that any type of slowdown
or downtime can mean the loss of millions of dollars. Clearly, just a bunch of
hard disks won't be able to cut it anymore. So Redundant Array of Independent
(or Inexpensive) Disks (RAID) was developed to increase the performance and reliability
of data storage by spreading data across multiple drives. RAID technology has
grown and evolved throughout the years to meet these ever-growing demands for
speed and data security.
A
technique was developed to provide speed, reliability, and increased storage capacity
using multiple disks, rather than single disk solutions. RAID takes multiple hard
drives and allows them to be used as one large hard drive with benefits depending
on the scheme or level of RAID being used. The better the RAID implementation,
the more expensive it is. There is no one best RAID implementation. Some implementations
are better than others depending upon the actual application. It used to be that
RAID was only available in expensive server systems. However, with the advent
of inexpensive RAID controllers, it seems it has pretty much reached the mainstream
market.
The
Array And Raid Controller Concept:
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. 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.
A raid controller writes the same data blocks to each
mirrored drive. This means that each drive or array has the same information in
it. We can add another level of complexity by introducing yet another technique
called striping. If we have one striped array we can mirror the array at the same
time on the second striped array. To set up mirroring the number of drives will
have to be in the power of two.
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