Introduction to NonStop Operations Management

Overview of NonStop Operations Management
Introduction to NonStop Operations Management–125507
1-9
Viewing Availability From an End-User’s Perspective
Using an Outage-Minutes-per-Year Measurement
Tandem recommends using a total outage-minutes-per-year measurement to reveal
outages. An outage is the time during which the system is not capable of doing useful
work because of planned or unplanned interruptions. From the end-user’s perspective, an
outage is any time an application is not available.
Using the outage-minutes-per-year measurement is easy to understand and provides
more meaningful data than percentile numbers such as “95 percent available.” Table 1-2
compares percentage numbers 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 down time 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
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 down time 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 down time. An
outage of one minute in the server, however, equals one minute times the number of
users accessing the server.
Table 1-1. LAN Availability and Down Time per 40-Hour Workweek (Traditional
Measurement)
Percentage of Time LAN Is “Up” Equivalent Number of Minutes LAN Is “Down”
90 percent 240 minutes
95 percent 120 minutes
99 percent 24 minutes
Table 1-2. 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.