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RAID is an acronym for Redundant Array of Inexpensive Disks. RAID is a technique
using two or more disk drives in combination for fault tolerance and performance.
RAID disk drives are used frequently on servers but aren't generally necessary for
personal computers.
There are number of different ways to use multiple disks together in a RAID
scheme and these are known as RAID levels. There are many different RAID levels,
and some manufacturers have developed their own variations. Although this can
make RAID confusing, the four levels of most interest to schools are 0, 1, 3, and 5:
- Level 0 provides data striping (spreading out blocks of each file across multiple
disks) but no redundancy. This improves performance, but does not deliver fault
tolerance.
- Level 1 provides disk mirroring, which gives good data protection but no
performance benefit.
- Level 3 is the same as Level 0, but also reserves one dedicated disk for error-
correction data. It provides good performance and some degree of fault
tolerance.
- Level 5 provides data striping at the byte level and also stripe error- correction
information. This results in excellent performance and good fault tolerance.
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The basic idea of RAID is to spread a server’s data across multiple disks
seamlessly. For example, a single file might be spread across four or five disks.
The RAID system manages all those parts so that when a file is opened the RAID
system accesses all the appropriate disks and reassembles the file.
The immediate benefit is that the multiple disks perform much more quickly than a
single disk. This is because all the disks work independently on finding their own
data and sending it to the controller to be assembled. A single disk drive would be
limited by a single disk head and would take much longer to gather the same
amount of data. In fact, the performance of a RAID system increases as you add
more disks to the array. You might think that by spreading data across multiple
disks you are increasing the chance of data loss due to disk failure. RAID
addresses this by creating checksums and error correction, so that if one or more
disks are damaged the data can still be retrieved. This makes for a very efficient
and cost-effective way to manage your data.
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The following guide will help you decide which RAID system is most appropriate for
your school:
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RAID system
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Features
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Main consideration
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Level 0
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Good performance but no protection
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Low cost
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Level 1
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Data duplication but no performance
benefit
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Data redundancy
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Level 3
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Good performance with some fault
tolerance
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Data protection
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Level 5
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Better performance plus good fault
tolerance
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Efficiency and
protection
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Pre-installation
- Configuration Choose the configuration that meets your performance,
storage, availability, and serviceability needs. Stick with standard
configurations for the best price/performance and ease of service, unless
your requirements dictate otherwise. Ask the difficult questions before
buying anything. The higher your availability requirements, the more
redundancy and component removability you require.
- Back up Ensure that your back-up device and procedures are adequate to
handle the increased storage in the time available.
- Power Ensure clean power and UPS protection.
- Interference Check for nearby sources of electromagnetic interference,
such as banks of modems on web servers.
- Load Is the system totally loaded already? Will the addition of one more
device, especially a high-performance RAID array, push the load limit over
the edge?
- SCSI interface Is the interface fast enough to avoid bottlenecking the new
high- performance equipment?
- Service Decide who will perform remedial service.
- Spare parts Do you need spare parts?
- Performance benchmark This is so that you can test how much faster
everything runs with the new high-performance storage. Time your longest
batch jobs and measure client response to lengthy transactions before and
after installation.
- Capacity assessment Based on performance benchmarks, you can make
a reasonable estimate of how many users you can support on the existing
server before you need to add another one.
During installation
- Scheduled downtime Make sure that there will be enough time to install
the system properly. Allow plenty of time and anticipate that it will take longer
than expected.
- Test period The more critical the application, the longer the new systems
need to bed in. Several days of running diagnostics, exercises and
representative applications is a reasonable precaution to take before
committing your entire school network to a new RAID system.
- Training Make sure one or (preferably) two or more people are trained on
the system. Training includes setting up RAID arrays, swapping
components, rebuilding arrays, simulating failures and, most importantly,
practising what to do in the event of actual failures.
After installation
- Protection of your data RAID is no different from any other storage
device when it comes to protection from viruses and accidental or
deliberate deletions. Implement the same file- protection and record-locking
strategies you would use on a non-RAID system.
- Data backup Typically, RAID systems significantly increase your total
storage capacity. Make sure you do a complete back-up frequently and
incremental back-ups at least daily. Test data restoration periodically to
ensure that you remember how to do it and to check that the tape system is
working properly.
- Data security Make extra back-ups and keep copies off site.
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