High Availability

If your business loses money when your network (or other system) is down, then you need a system with an appropriate level of high availability.

Availability is the percent of time that a system is expected to be fully functional. A typical, and not very good, value for the availability of a system might be 99% (twonines) Now, a year has approximately Y=8766 hours in it; thus, a system that is expected to be operational 24 hours per day, 7 days per week, and that has 99% availability would be expected NOT to be fully functional 87.66 hours every year! If a business loses $1,000 per hour when this system is down, then this translates to an expected loss of $87,660 annually due to system down time. Of course, some businesses lose less per hour but some lose much more! A system is (fully or partially) “down” if it is not fully functional. A first step in analyzing your system is to understand your causes of downtime.

A business that only operates, due to holidays, weekends, etc., say, 200 days per year and only needs its network to operate during 8 business hours per day, only has Y=1600 critical hours in a year. System maintenance and repairs can be done during off hours. This typically RAISES the availability of its systems. One can design appropriately high availability systems for such businesses at a much lower cost than for 7 by 24 systems.

We should point out that “expected” downtime and “expected” losses are averages taken over several years and over many similar systems. For a given system, they may well be more in one year and less in another year.

One approach to availability evaluations is to run two models. The first model estimates the true cost of downtime for your systems, and the second model computes the availability of your systems. This tells us, as in the above example, the expected annual loss due to down time. When this number is high, it makes sense to upgrade your systems to improve their availability and to consequently lower the expected annual loss. By modeling the proposed changes, you maximize your return on your investment in higher availability.


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