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Article ID : # Last review : 2007-04-20 16:42:00

What is uptime


Here's a little bit about 'Uptime'.

Firstly, in most cases, the uptime of single servers doesn't necessarily mean much to reliability when you are talking about major networks such as mail or dns with multiple redundant sites.

So, is 99.9% better than 99%. Is someone really saying their server has 99.999% uptime? The actual downtime for each '9' is listed below.

Uptime Corresponding downtime
98% 7.3 days
99% 3.7 days
99.9% 8 hours
99.99% 1 hour
99.999% 5 minutes

For any individual server, 99.999% is pretty unrealistic if you are maintaining it. Considering the reboot time alone is likely to exceed this, what about security patches etc that must be installed?

When we talk about the uptime of a 'service', this is completely different. Microtech publishes live uptime figures for all critical servers on the status page here. Most the uptime figures are conservative as if the monitoring server get's disconnected, even though the servers are up, the server counts some downtime as it cannot reach them. Still, most of our servers provide recorded uptime in excess of 99%. Does this mean we have 3.7 days downtime? No. Consider the dns network. There are a minimum of 4 sites. If a single site fails, dns resolution continues with almost no perceptible difference. DNS is a robust protocol and simply gets answers from the other servers. You could easily consider our dns uptime to be 100% based on these figures. The same follows for the mail servers. Simply because 1 server is down right now, the rest are still working, and the service, as a whole, is still running.

We can happily take down a dns server for maintenance. Yes, it will affect it's individual uptime, but it will not affect the dns service as a whole. Multiple servers in redundant locations are far more important, and in most cases, considerably cheaper to provide than the much sought after 5 9's uptime.

We hope this gives users something to thing about when considering uptime, especially as people sometimes consider it important in making service provider decisions.


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