You often hear vendors mentioning their system has five 9s of uptime, but what exactly is uptime?

Some define uptime only for their own specific piece of technology. For example, a storage array with five 9s uptime, can only tolerate 5 minutes and 15 seconds of downtime per year, but if your network vendor also has a five 9 uptime specification and your power company and your data center and your internet provider and a whole lot of other components…. do the math!

### Example

- storage array
- SAN
- Host (with virtual machines for example)
- data center Network
- data center power feed / power components
- internet / VPN / any connection to your office
- office network
- office power feed / power components
- your desktop pc / laptop

In my example there’s 9 consecutive failure domains. Suppose each “domain” has five 9s of guaranteed uptime, the math about the uptime for the end-user is:

0.99999ˆ9 (to the power of) = 0.99991

But let’s face it: not all components actually have five 9s uptime.

### How much downtime is five 9s availability per year?

Well, this list shows how much downtime a certain number of nines is:

- 99.999% = 5 m 15 sec
- 99.99% = 52 m 36 sec
- 99.9% = 8 h 46 m
- 99% = 87 h 40 m –> I don’t know any component that has this much downtime per year

Let’s make a few assumptions:

- storage array: 99.999%
- SAN: 99.999%
- Host (with virtual machines for example): 99.99%
- data center Network: 99.99%
- data center power feed / power components: 99.999%
- internet / VPN / any connection to your office: 99.9%
- office network: 99.95%
- office power feed / power components: 99.95%
- your desktop pc / laptop: 99.5%

The uptime for this specific example is:

99.999% x 99.999% x 99.99% x 99.99% x 99.999% x 99.9% x 99.95% x 99.95% x 99.5%

This equals to 99.27% uptime in total and this equals to * 63* hours of downtime per year.

As you can see that the weakest link weighs heavily on the amount of downtime per year. Improving uptime for a component that already has five 9s (Storage, SAN and DC power) makes the downtime go down to * 62.4* hours.

But if the uptime of the desktop is improved from 99.5% to 99.9% the downtime goes down from 63 to 28 hours!!! Wait, WHAT?

So improving the weakest link has a much larger impact on uptime than hardening the toughest product?

### Conclusion

In my opinion improving from five to 6 or even 7 nines uptime for a single component, when the whole “network” of component still has serious flaws, has little to no effect. It’s better to invest in buying better products that have 3 instead of 2 nines uptime (or similar).

In my opinion, to be precise, 99,999% uptime can mean 5,26 minutes downtime :-).

5.26 minutes = 5 minutes and 15.6 seconds. I rounded it to 5 min and 15 sec.