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I have been wondering this for quite a while. Let's say that you have a Debian server that you keep updated via APT. Usually (every 1-2 months) there are some kernel updates, which will update the GRUB entries to boot from it next time.

This is okay, but if you care about uptime and SLA, it would be difficult to reboot just to use the updated kernel. I guess this is the usual way to go, but is it actually how it works?

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Don't forget rebuilding the virtualization suite tools. –  galegosimpatico Dec 10 '13 at 14:38
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The only method I've ever heard of that allows you to forgo performing a reboot is through the use of the KSplice technology.

excerpt

What is Ksplice Uptrack?

Ksplice Uptrack lets you apply 100% of the important kernel security updates released by your Linux vendor without rebooting.

Ksplice Uptrack is available for Oracle Linux, free of charge, for Oracle Linux customers with a Premier support subscription. Additionally, anyone can use Ksplice Uptrack for free on Ubuntu Desktop and Fedora.

According to the website it's free for Fedora and Ubuntu Desktop users:

excerpt

Free for Fedora and Ubuntu Desktop.

History of technology

The KSplice technology came on the scene in 2009, if memory serves me correct. The impressive demo the technology developers used was to migrate a Linux system through all of the Linux Kernels that were ever released. I never saw the demo but that claim has stuck in my mind ever since.

Here's the Ars Technica article that I remember reading, titled: Ksplice: Update computers without rebooting.

Lastly here's the technical paper that covers the technology and how it works, titled: Ksplice: Automatic Rebootless Kernel Updates.

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Never heard of that, that's fascinating! +1 Have to find that demo... –  chaos Dec 10 '13 at 15:22
    
There are some demos on youtube, these are new ones not of the full upgrade: youtube.com/watch?v=1m5HJ7h1l7I –  slm Dec 10 '13 at 15:46
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It's not necessary in the sense that you must, it's only necessary if you want to use the new kernel right away.

Simply updating should never leave the system in a state whereby you must reboot after the update. One obvious reason for this is a lot of systems update automatically, and an automatic update that screwed the running system would not be appreciated by many users.

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Indeed, I am aware that you do not actually "must" do it if you do not need the new specific features/fixes of the updated kernel, however in most of the cases it is a recommended action. –  Raj Dec 10 '13 at 14:39
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Why and by whom? Being slightly paranoid about software glitches is reasonable, and so if you have no reason not to reboot, then you should reboot, in case there is a glitch in the upgrade (of course, if there's a glitch, it could just as easily be a glitch that won't get exposed until a reboot, so...). However, if you have a good reason not to reboot, then don't. Of course, this leads to the obvious: if you have a good reason not to reboot, then don't upgrade until you can. –  TAFKA 'goldilocks' Dec 10 '13 at 14:46
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Yes, to use an kernel update, you usually need to reboot the system. Sometimes it could be enough, to reload a kernel module - but this is difficult to decide.

By using kexec, you can shorten the time the system is not available.

Ksplice offers for some distributions kernel live patching.

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Is necessary? Yes, if you want the new kernel. But it can be worked around? Yes too. But then, why do we need reboot to upgrade the kernel? Because you don't reboot for anything else! Now, for the workaround you should be aware that some weirdness can (and will) happen.

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