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Redundant
Disk System - RAID |
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| RAID Level |
Description |
Performance
Advantage |
Fault Tolerant? |
| RAID
0 |
Disk Striping |
Parallel Disk I/O |
No |
| RAID
1 |
Disk Mirroring |
None |
Yes
(1 Drive Failure) |
| RAID
2 |
Disk Striping with Hamming Code for Error
Protection |
None |
Yes
(1 Drive Failure) |
| RAID
3 |
Disk Striping with Dedicated Parity Drive Parallel Disk
I/O |
Yes |
(1Drive Failure) |
| RAID
4 |
Disk Striping with Dedicated Parity Drive; Non-
synchronized Disks Required |
Parallel Disk I/O |
Yes
(1Drive Failure) |
| RAID
5 |
Disk Striping with Distributed Parity |
Parallel Disk I/O (not as Fast as RAID 0) |
Yes | |
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Redundant Arrays of Independent Disks, or RAID, is a
rapidly expanding storage technology which promises a major
improvement in the way on-line data is stored in
computers.
RAID Level
Definitions
Those investing in
storage will need to consider low cost per Mbyte, high input/output
I/O, and high data reliability in order to obtain a balance to suit
their needs.
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RAID 0 |
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RAID 0 - Disk Striping
Disk striping writes data across all disks concurrently
rather than on one disk at a time. Although termed RAID 0, it is not
a true implementation of RAID because there is no facility for
redundancy. Therefore, in the event of a disk failure, data is
lost.
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In the disk array subsystem, data chunk 0 is written to
disk 0 , chunk 1 is written to disk 1 and so on. When the last disk
is reached and written, the array proceeds to store data on the next
level of the first disk.
Disk striping is fast as data can be
transferred to multiple disks simultaneously: chunk 0 is still being
written to disk 0 while chunk 1 is being written to disk 1.
Furthermore, reads and writes can overlap.
An example of a
typical usage for RAID 0 could be: Data from the field comes into
the central processing location on tape where it is instantly
processed. Redundancy is not a requirement as the tape can be
relocated.
Summary: RAID 0 offers the highest performance without
redundancy. Some industries that RAID 0 is particularly suited to
are: meteorology, geophysical exploration, oil and gas industries,
video/graphics.
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RAID 1 - Disk
Mirroring
Disk mirroring protects against disk failure
by keeping two copies of data stored on separate disks or arrays.
Though simple and easy to implement, installing two sets of disks
effectively doubles the investment required for a single,
non-redundant drive. If at any time either disk fails, the remaining
disk can provide all of the data needed, preventing downtime.
Two
copies of the data also ensure that there is no degradation in
performance, as accesses are immediately routed to the working disk.
In the event of failure, copying from the operational disk to the
replacement disk is very fast, which reduces the risk of a second
failure.
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RAID 1 |
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RAID 1 not only provides protection, it can also
improve performance. For example, if multiple requests for the same
data are made, demand can be distributed between two disk copies
therefore increasing response time for data access.
Summary: RAID
1 is the most secure of any of the RAID levels and is exceptionally
fault-tolerant. Examples of industries that would use this level are
those who cannot afford downtime: banks, insurance companies, stock
markets, airline systems.
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RAID 3 |
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RAID 3 - Parallel Data
Access
In RAID 3 data is
distributed to a striped array and a disk is added to store
redundant information. The array consists of three disks for the
data and one parity disk for the redundancy. In the event of a disk
failure, data can be mathematically reconstructed from the remaining
disks in the array.
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Synchronization enables striped data to be read and
written as quickly as possible. However, when multiple writes are
involved, performance is reduced because the parity drive has to be
accessed for every single write, which may create a bottleneck at
the parity drive. Consideration should also be given to impacts on
performance as disk rotation must be synchronized before data can be
accessed.
Summary: RAID 3 is ideal for intensive high-speed, long
data transfer applications such as: video, CAD/CAM, graphic
applications, scientific modelling.
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RAID 5 - Independent Access
Arrays
In RAID 5, the redundancy
offered in RAID 3 by a single parity disk, is distributed across all
the disks in the array. Data and relative parity are never stored on
the same disk.
One user may be writing a chunk to disk 0 and the
corresponding parity to disk 3, another user may be writing to chunk
4 of disk 1 and updating parity on disk 2. There is a clear dividend
in terms of performance and the speed of
transactions.
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RAID 5 |
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During disk writes, RAID 5 cannot produce a write
performance comparable to that of straight disk striping because
other operations have to be undertaken to make and store parity
codes. The I/O performance of the array depends very much on the
relative levels of reads and writes requested.
When a stripe is
modified, unmodified portions must also be read to re-generate the
parity for the entire stripe. Once the parity has been generated,
the modified data and parity information must be written to disk.
This is commonly know as Read/Modify/Write strategy.
It reflects
that, though RAID 5 is superior to RAID 0 because it offers
redundancy, it is not able to perform as well as RAID 0 in terms of
write performance. Because RAID 5 has distributed parity, two reads
and two writes must be performed for every write operation. However,
the write penalty can be overcome by the use of write caching which
allows write data to be stored in the memory prior to writing to the
disk, so freeing the host processor for other tasks.
Summary:
RAID 5 is ideal for organizations running databases and other
transaction-based applications such as: banks, airline and railway
reservation systems, government departments, utilities and
telecommunications.
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