NonStop NS-Series Planning Guide (H06.03+)

Planning for System Availability and Maintenance
HP Integrity NonStop NS-Series Planning Guide—529567-004
8-2
How Availability Is Measured
downtime caused by a problem situation such as an AC power outage, faulty system or
application software, faulty hardware, operator error, disaster, and so forth.
How Availability Is Measured
HP believes that availability should be measured from the end user’s perspective.
Simply recording that a certain hardware or software component is not operating is not
enough. You must also consider the user’s ability to access the service, the quality of
the service provided, and the acceptability of the response time to the user.
Although major changes—such as installing a new operating system—obviously affect
availability, the effect of other types of changes might be less apparent. For example,
changing the characteristics of a communications line could cause response time to
become unacceptable to an end user who is trying to use that line to access a file on a
remote system while the change is being performed.
Although the computer industry has typically measured availability in percentages, HP
recommends measuring availability by outage minutes in 24-hour-a-day, 365-day-a-
year operations.
Outage Minutes/Year
An outage-minutes per year measurement is easy to understand and provides more
meaningful data than percentile numbers such as 95 percent available.
Minimizing Planned Outage Frequency
By taking the time to anticipate and plan for changes, you can avoid taking your
system down for unnecessary planned outages.
You can take action now to prevent planned outages in the future in these areas:
•
Evaluating System Performance and Growth on page 8-3
•
Providing Adequate Computer-Room Resources on page 8-3
•
Configuring Your System to Accommodate Future Changes on page 8-3
•
Implementing a Formal Change-Control Process to Manage Change on page 8-4
Percent Availability
90% 99% 99.9% 99.99% 99.999% 100%
Outage
Minutes/Year
*
50,000 5,000 500 50 5 0
User Impact
*
35 days 3.5 days 8.3 hours 50 minutes 5 minutes 0 minutes
*These impacts are approximations.