Databases Reference
In-Depth Information
Table 11-1. Percent availability and downtime per year
% availability Downtime per year
Days HoursMinutes
95.000
18
6
0
96.000
14
14
24
97.000
10
22
48
98.000
7
7
12
99.000
3
15
36
99.500
1
19
48
99.900
0
8
46
99.990
0
0
53
99.999
0
0
5
Large-scale systems that achieve over 99 percent availability can cost hundreds of thou‐
sands of dollars or more to design and implement and can have high ongoing opera‐
tional costs. Marginal increases in availability can require large incremental investments
in system components. While moving from 95 to 99 percent availability is likely to be
costly, moving from 99 to 99.999 percent is usually costlier still.
You might also consider
when
the system must be available. A required availability of
99 percent of the time during normal working hours (e.g., from 8 a.m. to 5 p.m.) is very
different from 99 percent availability based on a 24-hour day. In the same way that you
must carefully define your required levels of availability, you must also consider the
hours during which availability is measured. For example, many companies take orders
during “normal” business hours. The cost of an unavailable order-entry system is very
high during the business day, but drops significantly after hours. Thus, planned down‐
time can make sense after hours, which will, in turn, help reduce unplanned failures
during business hours. Of course, in some multinational companies, the global reach
implies that the business day never ends.
A requirement that a database be available 24/7/365 must be put in the context of the
cost in deploying and maintaining such a solution. An examination of the complexity
and cost of very high levels of availability will sometimes lead to compromises that
reduce requirements for this level of availability.
The costs of achieving high availability are certainly justified in many cases. It might
cost a brokerage house millions of dollars for each hour that key systems are down. A
less-demanding use case, such as human resources reporting, might be perfectly fine
with a lesser level of availability. But, regardless of the cost of lost business opportunity,
an unexpected loss of availability can cut into the productivity of the lines of business
and IT staff alike.
