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by Alex Young, Director of Technical Marketing Europe,
Infortrend -
December 2006 |
| RAID technology has
enabled users to get much more out of their hard disk drives, namely data
protection, fault-tolerance, increased performance and larger capacity. In a
RAID array, which can be configured to different levels e.g. 0, 1 or 5, data is
shared and/or replicated across multiple disks. Below are the top 10 tips which
existing and potential users of RAID technology should consider for their
implementation of the technology:
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| 1 |
Compare like-for-like
When
comparing RAID products don't focus on the CPU clock speed as that doesn't
always mean faster performance. You should look at the performance figures
(MB/sec or IOPS) instead.
Only when two RAID products are based on
identical RAID ASIC, RAID firmware, and hardware design, then comparing the CPU
clock speed might make sense. Each RAID product manufacturer offers different
CPU, architecture, firmware, hardware, RAID ASIC, etc, hence the CPU clock speed
comparison alone is not a reliable clear performance comparison criterion.
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| 2 |
Keep it flowing
If you have purchased a 24- or 16-bay RAID subsystems to meet future growth and
have not installed disk drives in each bay, install empty trays in the
chassis as this will ensure sufficient air flow.
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| 3 |
Needle in a haystack
If you, like many RAID users, have deployed dozens or even hundreds of RAID
subsystems, let the software display a text string that the administrator can
identify when searching for a specific unit.
For example you can use
the IP address, the name of the connecting host computer, or you can even give
the RAID subsystem a name or number.
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| 4 |
Ensure you have enough
capacity
Before proceeding with a RAID migration project make sure
you have sufficient free capacity or unused drives in your RAID subsystem.
RAID
6 arrays require at least four member drives and use additional capacity for the
distribution of secondary parity.
For example if you decide to migrate
a three-drive RAID 5 array to RAID 6 you will need one additional disk drive or
enough unused space to have a second parity.
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| 5 |
Protect the cached data
The
life span of a battery varies according to the number of recharging /
discharging cycles hence you should replace the Battery Backup Module (BBU)
after 12 months of operation in order to safeguard the cached data should a
mains power failure occur.
Editor's note:- future RAID controllers
may use PRAM, SRAM or MRAM which don't need batteries.
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| 6 |
Check your writes
The
"Media Scan" should be performed regularly. Unless you enabled the "Write-Verify"
function for the normal writes, the disk drives usually do not verify the data
when writing.
Performing "Media Scan" can decrease the risk
of having multiple data blocks missing, and lowers the risk of data loss.
You
can have the RAID subsystem perform the "Media Scan" monthly by using
the automatic scheduler function. |
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| 7 |
Reduce latency
When
the cache Write-Back is disabled (Write-Through Mode) the entire host IOs are
passed directly to the disk drives after RAID operations.
All the disk
drives will be accessing the data blocks in an order related to the host, and
most of the time will be moving the read/write arm and waiting for the data
blocks (the so called Latency Time).
When the cache Write-Back is
enabled, the Write data from the hosts are collected in the cache memory,
optimised with the cache algorithms and then flushed to the disks by the RAID
controller. The Write-Back cache mode does save a big percentage of disk drive
latency time and provides a much better Write performance in most
situations, compared to the Write-Through mode (Write-Back disabled).
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| 8 |
Beware of slow PCI slots
You might be struggling to reproduce the highest performance figure of the RAID
subsystem. This might be due to the host computer slots in which the
SCSI or
Fibre Channel HBAs are
installed.
Often there will be only one or two available PCI slots in
a computer and while other PCI slots might look the same they might be running
at lower speeds. Depending on the computer's internal design, in many situations
multiple PCI devices will have to share the PCI bandwidth and these will all
limit the maximum performance that can be performed by the HBA, and affect the
performance test results.
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| 9 |
Plan for growth
When
creating RAID Logical Drives, plan to accommodate any future drive capacity
variation and be aware that drives from different manufacturers which are
supposed to offer the same capacity will actually vary in size.
The
capacity of the disk drive is measured by the numbers of available data blocks,
often labelled on the drive as "LBA" (Logical Block Address, each
block is 512 bytes).
In a RAID Logical Drive all member drives will be
used up to the maximum common denominator so if for example three disk drives in
a RAID 5 Logical Drive, have 100, 99 and 101 blocks, only 99 blocks will be used
on each disk drive when creating the RAID 5 onto these three disks.
This discrepancy means that when a drive has failed and the replacement drive is
slightly smaller in capacity the "rebuild" won't start. The
solution? When you create the RAID Logical Drive use 1% less capacity than the
'official' one.
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| 10 |
Hard copies not just
hardware
Keep a print copy of the RAID array configurations and
connection schemes as in some situations if the complete system is being
replaced and the replacement is not the same product (e.g. from different
vendors), the original configuration file might not be work.
Keeping a
hard copy of these details can ensure you quickly have the new unit up and
running.
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About the author:-
The
author, Alex Young, is Director of Technical and Marketing EMEA at
Infortrend |
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