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I am using webmin (which uses yum) to install updates on my server, and it somestimes updates kernel as well including kernel-firmware and kernel-headers. Do I need to restart the server after a kernel update?

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If you're interested in using the kernel, yeah. – NickW Apr 26 '13 at 11:26
Here's my motivation for not reopening. This site is for professional sysadmins. If you don't know the basics of how updating a kernel works, you are not at that level. You need to at the very least read the docs for the system you're working on, possibly get some training. Questions of this level can be answered with some basic googling, or searching at the unix/linux sister site. They're not what this site is for. – Jenny D May 16 '13 at 9:18
up vote 16 down vote accepted

There are at least two reasons for rebooting:

  1. You probably want to use the advantages of the newer version (security fixes)
  2. Usually during a kernel update the module tree of the old kernel is removed. Thus if you (or some script) unload a module then the system cannot load it again because it finds only the newer one on disk (if at all) and this is compiled for a different kernel and thus cannot be loaded (at least usually).
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Good point about the modules. Some distros can handle this (keeping old and new versions), other don't. – Tonny Apr 26 '13 at 12:02
@Tonny Now you know which distro I use... :-) – Hauke Laging Apr 26 '13 at 12:05
@Tonny and HaukeLaging: Which distro can do that? That's wonderful. – kmort Apr 26 '13 at 13:04
@kmort: I think Hauke is using one that can't ^^ And that's why he thought about this in his answer... :p – Olivier Dulac Apr 26 '13 at 15:58
@Hauke: +1 for the module tree warning. – Olivier Dulac Apr 26 '13 at 15:59

To actually use the new kernel you will have to reboot.
No way around that.

Do you have to reboot immediately after installing the update ?
In general NOT, you can schedule the reboot for a more convenient moment.

But it is NOT a good idea to install more updates before you reboot.
Some updates have different versions depending on which kernel you are running.

And anyway: After rebooting with a new kernel always check for updates. Some of your existing packages may need a new version after the kernel update.

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It depends.

Ksplice takes care of all security updates but it depends on the updates and it's not free. You can check here for details for yum as well.

You also have the option of using kexec which will require a reboot but will skip the hardware part of the boot.

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