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I want to know the standard of web server, file server, database server and print server downtime per day.

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There's no such thing as a standard - there are varying uptime guarantees - usually specified as a certain amount of 9s (e.g. five 9s is 99.999%, and corresponds to 5.26 minutes downtime a year - about 0.86s a day). See this Wikipedia article on High Availability.

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As Dominic says, no such thing as a 'standard'. There's "what the organisation is willing to pay for" - improvements in uptime (e.g. to go from 99% availabilty to 99.9%) require improvements to at least one (and often several of) staff levels and knowledge, working practice, amount and configuration of equipment. If you invest with an eye to improving uptime then you can do it, but when you get to 99.999% availabilty you really are talking about having implemented things like clustering, with cluster members in geographically seperated datacentres. Do-able, but not cheap.

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Like others have said, there's no standard for downtime. Ideally it's 0... but in the real world, people pay for certain availability. They make Service Level Agreements (SLAs) about what they expect from the service. If these expectations aren't met, there will be an agreed upon compensation.

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Uptime requirements will be different for different systems, and will be influenced by the business requirements. It might be a great thing to day that you have 5 9s, but are you putting a lot of work into ensuring uptime during periods when the servers don't actually need to be up?

Example: you mention a file server as one of the types you're interested in. Now, a day is 24 hours, and assuming that 8 hours of those are the working day, you could in theory get away with 66.66666... % downtime!

Likewise with a print server, what is the business impact if you take it down for 5 minutes every couple of hours? Depending on the volume of printing and what's being printed, there could be no business impact at all - and yet it would be measured against your overall downtime!

So no, absolute measurements of downtime according to some hypothetical standard are actually quite meaningless. Better to measure in terms of actual business impact instead.

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I try not to think of my servers but more of the service they provide. My servers are down often, once a month at least for patches, while the service they provide is little if not zero - this is achieved via a combination of clustering and load-balancing, fairly basic tricks that ensure you can have your cake and still eat it :)

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