Service Level Agreements from companies usually explains that the product they are supporting will be up or available for X% of the time. 90% is good, 99% is even better, and some go on up to 99.99% or higher, but what does that mean for a business in plain terms?
|
Availability % |
Downtime per year |
Downtime per month* |
Downtime per week |
|
90% |
36.5 days |
72 hours |
16.8 hours |
|
95% |
18.25 days |
36 hours |
8.4 hours |
|
98% |
7.30 days |
14.4 hours |
3.36 hours |
|
99% |
3.65 days |
7.20 hours |
1.68 hours |
|
99.5% |
1.83 days |
3.60 hours |
50.4 minutes |
|
99.8% |
17.52 hours |
86.23 minutes |
20.16 minutes |
|
99.9% (“three nines”) |
8.76 hours |
43.2 minutes |
10.1 minutes |
|
99.95% |
4.38 hours |
21.56 minutes |
5.04 minutes |
|
99.99% (“four nines”) |
52.6 minutes |
4.32 minutes |
1.01 minutes |
|
99.999% (“five nines”) |
5.26 minutes |
25.9 seconds |
6.05 seconds |
|
99.9999% (“six nines”) |
31.5 seconds |
2.59 seconds |
0.605 seconds |
* Downtime per month is calculated at 30 days.
Please note as the Availability Percentage goes up it sometimes becomes cost prohibitive or exponentially more expensive. You can also have different levels of availability on different components. For example your e-mail servers might be more important than an internal web site so you spend more to keep them up and flowing. If you are a mid size business with several servers then part of trying to get to the higher uptime means that you might have to purchase redundant servers so if one is down for maintenance then the other one is up and working. Sure you add fault tolerance with a second location that mirrors the same dual servers for the first one – but it could go on and on until you have spent more money and more money just to get to those extra 9’s. For some businesses like Wal-Mart and Amazon this turns into money well spent since you are losing millions depending on the downtime. Make sure you take a look at your needs and abilities that way you can set your expectations appropriately.
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