| A BRIEF
OVERVIEW OF RAID |
Understanding the basic concepts of RAID
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Concept of RAID
RAID (Redundant Array of Independent Disks)
is an acronym first used in a 1988 paper by
Berkeley researchers Patterson, Gibson and
Katz. It described array configuration and
applications for multiple inexpensive hard
disks, providing redundancy and better
access rates. When creating a RAID it makes
a method of accessing multiple individual
disks as if the array were one larger disk.
It spreads data access out over these
multiple disks, thereby reducing the risk of
losing all data if one disk fails, and
improving access time.
Why RAID?
Typically RAID is used in large file
servers, transaction of application servers,
where data accessibility is critical, and
fault tolerance is required. Recently
desktop users have started to use RAID for
almost any application where massive storage
or high data transfers are needed.
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RAID Levels |
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RAID 0 |
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Disk Striping, data is written in
blocks across multiple disks, this enables
you to read from one disk while writing to
another. Technically this is not a RAID
since it does not provide fault tolerance.
The advantages of striping are faster access
rate, and full utilization of the array
capacity in a centralized manner. The
disadvantage is there is no fault tolerance
- if one disk fails the entire contents of
the array become unavailable. This array
requires at least 2 disks. |
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RAID 1 |
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Disk Mirroring, provides redundancy
by writing the data to both disks,
once to each disk. If one disk
fails, the other contains a copy of
the information and the RAID can
switch to using the mirror disk with
no lapse in user accessibility. The
disadvantages of mirroring are no
improvement in data access speed,
and higher cost. However, it
provides the best protection of data
since the array you will always have
a copy of the data. This array
requires at least 2 disks |
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RAID 3 |
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RAID 3 stripes data across multiple
disks, with an additional disk
dedicated to parity, for error
correction/recovery. This array
requires at least 3 disks. |
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RAID 5 |
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RAID 5 is the most popular
configuration, providing striping as
well as parity for error recovery.
In RAID 5, the parity block is
distributed among the disks of the
array, giving a balanced access load
across the disks. The parity
information is used to recovery data
if one disk fails, and is the reason
this method is the most popular. The
disadvantage is a slow write cycle,
2 reads and 2 writes are required
for each block written. The array
capacity is the amount of disks
minus 1. This array requires at
least 3 disks. |
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RAID 0+1 |
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This is stripping and mirroring
combined, without parity. The
advantages are fast data access
(like RAID 0), and a single 4 disk
fault tolerant system (like RAID 1).
RAID 0+1 still requires twice the
number of disks (like RAID 1). This
array requires at least 4 disks.
Software vs. Hardware RAID
For RAID to function properly, there should
be software either through the OS or through
dedicated hardware to properly handle the
requests from the computer system to the
drive array. This is important since the
large amount of processing is required in
order to create the parity calculations
within RAID 5.
When implementing software RAID, the
processor is under heavy load to perform the
task that RAID requires, which can cause
lower overall computer performance for
increased disk performance or fault
tolerance. Software implementations are
relatively cheaper then hardware
implementations of RAID. The issue with
software RAID implementations is that it
puts the processor under heavy load The
processor load can be as low as 5% depending
on system configurations, such as Processor,
Memory or the amount of drives in the array
along with the type of RAID in the chosen
configuration.
Hardware RAID uses dedicated circuitry to
handle the strenuous processor it requires
for the calculations of the RAID Array. This
provides superior performance over software
RAID for the array. The main disadvantage in
hardware raid is the cost, although recently
costs of RAID controllers have been
decreasing. |
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