Assuming hardware failure is not a factor, and the requirement of being able to update periodically, is it possible to never shutdown Linux?

I typically do a full reboot after updates, especially kernel updates, but is there a way to keep my machine on and still do these? People always hear about incredible up-time, but how is that really possible if you must reboot after major updates.

Maybe a different run level? But then how would the kernel update kick in?

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    See also: ksplice, pannus. – jscott Apr 7 '13 at 10:52
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    "Incredible uptime" is a synonym for "my box is begging to be compromised, if it's not already." As your question implies, those machines with incredible uptimes generally haven't been patched. – HopelessN00b Apr 9 '13 at 16:17

Server/Box "uptime" is an illusion. Unless your objective is to have incredible uptime in order to prove some kind of point then I wouldn't focus on it.

What matters is service availability. If you need a service to be available all the time then it might be useful to improve individual system uptime or it may well be simpler and more cost effective to create a cluster, for example, than to try and take the availability of a commodity server from 99% to 99.999%

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    +1 Couldn't agree more. The idea that individual box reliability should be related to service availability and reliability is a fallacy. – Dan Apr 7 '13 at 16:18
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    Good work not answering the question. Why is this even the accepted answer? It should be downvoted for taking a detour. At the very least you should have tacked your current answer onto something that actually explains if OP's question is possible or not. – atx Apr 8 '13 at 8:05
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    @atx feel free to downvote me if you wish - it's the accepted answer because the OP accepted it. And yes, I didn't address the question in the title because I instead chose to address the point about up-time in the question body. After all, Alojz's answer addresses the other points quite well and it would be senseless to duplicate their post. – Rob Moir Apr 8 '13 at 12:57
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    @RobM Ah, fair enough then. I didn't look at his post. – atx Apr 9 '13 at 0:07

A standard linux is not designed to do this. It tries to minimze the reboots, but on some updates, like switching the kernel, reboot may be required.

There are projects that aim to also avoid this. See http://www.ksplice.com/ - now Oracle.

For live patching there is pannus, see http://lists.linux-foundation.org/pipermail/live_patching/attachments/20050725/14611ec4/050726pannus_latest.pdf

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