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Comment by TamDenholm | Hacker News Reader

I wonder if its cheaper (and/or better) for a hosting provider to spend less on reliability and instead build in multiple redundancy. Thoughts?

Also, i've never heard the expression 4 nines before, anyone explain that for me?

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It's a common way to describe uptime. It means 99.99% uptime/availability, which works out to under 1 hour of downtime per year.

http://en.wikipedia.org/wiki/Nines_(engineering)

Multiple Redundancy IS reliability. As long as everything stays running from the customer/business perspective, you are reliable.

When you start to define reliability as a specific layer of your infrastructure, you are setting yourself up for problems. You should be redundant at all layers, thereby reliable across your entire stack.

4-nines is 99.99% reliability (downtime of about 1 hour per year). More detail :http://en.wikipedia.org/wiki/High_availability

The number of nines refers to how reliable something is. "Four nines" means that it is up 99.99% of the time, or 0.01% of downtime, which is 0.0001 * 365 * 24 = just shy of 1 hour of downtime per year (or 1 day downtime every 30 years).

These things are usually specified in SLA (service level agreements) where if the provider exceeds the amount of downtime in the SLA means that you don't have to pay. If Amazon has a 4-nines SLA that means that you don't get any discount if they have 1 hour down per year.

> I wonder if its cheaper (and/or better) for a hosting provider to spend less on reliability and instead build in multiple redundancy. Thoughts?

Wouldn't it look the same from my end?

I assume it refers to uptime, e.g. 99.99% uptime.

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