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B.3
RAID Rules
When connecting devices and configuring them to work with a RAID controller, follow this set of guidelines:
•
To lessen their impact on the SCSI bus, connect SCSI-based tape, CD-ROM and other non-disk devices on a single
channel, which preferably does not service any disk drives
•
If using more than two drives, distribute them equally among all the channels on the controller. This results in better
performance. Depending upon the model, disk array controllers have between one and five channels
•
A maximum of 16 devices can comprise a Drive Group.
•
A Drive Group can contain devices on any channel.
•
Include all drives of the same capacity in the same Drive Group.
•
If configuring for a standby or hot spare drive, make sure the standby drive size is greater than or equal to the
capacity of the smallest drive in all the redundant Drive Groups.
•
When replacing a failed drive, make sure the replacement drive size is greater than or equal to the size of the
smallest drive in the effected Drive Group.
B.4
CAP Strategies Overview
Capacity, Availability, and Performance (CAP) should characterize your expectations of the disk array subsystem.
What is the best strategy for configuring the array? Do you want to access the maximum capacity of your disk drive
investment? Or do you want to configure your array for a high degree of fault-tolerant operation? Perhaps performance
is extremely important to you. Every installation has a different set of requirements, and most installations have a
combination of these.
Unfortunately, these options are somewhat mutually exclusive and it isn't possible to configure an array with all of these
characteristics optimized. For example, you can't have maximum capacity and maximum availability in a single array.
Some of the disks must be used for redundancy, thus, capacity is reduced.
Similarly, configuring a single array for both maximum availability and maximum performance is not an option (unless
you own a very large quantity of disk drives).
You will need to make compromises. Fortunately, your disk array controller is versatile enough to offer any of these
preferences, either singly or in the most favorable combination possible.
Bottom line: You need to decide which capability is most important for your operational environment, and which is
second most important. Once you've prioritized your expectations, it should be relatively easy to configure the array to
meet your expectations. To help you decide, complete the CAP questionnaire.
B.4.1
CAP Questionnaire
Before configuring the array, complete the following CAP strategy questionnaire. After completing this questionnaire,
take the information and use it as a guideline when choosing your configuration method.
I have ____ disks for the array.
Rank the following statements in order of importance: 1, 2, and 3
I want to emphasize capacity____ (see B.2 RAID Levels on page 214
I want to emphasize availability _____ (see B.2 RAID Levels on page 214)
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Spheras Storage Director Installation and User Guide
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