Availability Guide for Change Management

Introduction to Change Management
Availability Guide for Change Management–125506
1-7
Measuring Outages by Outage Minutes
ability to access the service, the quality of the service provided, and the acceptability of
the response time to the user.
While major changes—such as installing a new operating system release—obviously
affect availability, the effect of other types of changes may be less apparent but can also
affect end-user availability. For example, changing the characteristics of a
communications line could cause response time to become unacceptable to an end-user
who is trying to access a file on a remote system at the same time that the change is
being performed.
Measuring Outages by Outage Minutes
While the computer industry has typically measured availability in percentages, Tandem
recommends measuring availability by outage minutes, assuming a 24-hour by 7-day by
year-round clock. Using an outage-minutes-per-year measurement is easy to understand
and provides more meaningful data than percentile numbers such as “95 percent
available.”
Table 1-1 compares percentages with equivalent outage minutes and the resulting user
impact.
Measuring User Outage Minutes in a Client/Server Environment
For client/server types of applications, it is useful to express downtime as the number of
user outage minutes. A failure in the client part of the application might affect only one
user; but to that user, the application is down. A failure in part of the network could
affect several users. A failure in the server, however, could affect thousands of users. It
is therefore important that an outage in the server be weighted over an outage in the
client.
In a client/server environment, it therefore makes sense to measure downtime as the
number of minutes the application is unavailable multiplied by the number of affected
users. A one-minute outage in the workstation equals one minute of downtime. An
outage of one minute in the server, however, equals one minute times the number of
users accessing the server.
Alternate Ways of Measuring Outages
Depending on specific business needs, downtime may be measured in ways other than
user outage minutes. For example, a site might be obligated to pay a penalty for each
transaction that does not get processed while an application is down. Such a site might
Table 1-1. Outage Minutes per Year (24-Hour by 7-Day by Year-Round Clock)
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
*Outage minutes per year and user impact days are approximations.