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When upgrading the kernel package on CentOS or RHEL 6, is it possible to load the new kernel without rebooting?

My gut tells me it's not, but if it were, it'd be handy - especially when compiling applications that require kernel modules (like VirtualBox, for example) that complain when the "wrong" kernel source is available (eg the running kernel is 2.6.2-100 but the upgraded one is 2.6.2-152).

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4 Answers

up vote 3 down vote accepted

No.

kexec will allow you to skip the firmware (BIOS/EFI) and bootloader steps of a standard restart, but this is not the same thing (though useful if you have a slow loading RAID card).

The closest you can get is loading and unloading kernel modules.

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that's a shame :| .. thanks, though :) –  warren Jan 18 '12 at 23:06
    
"loading and unloading kernel modules" - can you actually load kernel-modules of a newer kernel? Sounds tricky to me. –  Nils Mar 7 '12 at 20:17
    
Didn't mean to imply the loading of modules from newer kernel, just modules that may not have been built on the same version. –  84104 Mar 7 '12 at 21:05
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Oracle Enterprise Linux can do that with KSplice, but you have to have an oracle ULN (support) contract for it. I believe they are about $120. http://www.ksplice.com/pricing

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KExec claims that you can do exactly that.

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how does kexec work, and can it be automated to fire as soon as a kernel is upgraded? –  warren Jan 18 '12 at 22:17
    
From the link @monksy gave: "Essentially, kexec is a fast reboot feature that lets you reboot to a new Linux kernel -- without having to go through a bootloader." No mention of it being automated, it seems unlikely based on this statement: "Unlike the normal reboot process, kexec does not perform a clean shutdown of the system before rebooting. It is left to you to kill all applications and unmount file systems before attempting a kexec reboot." –  codewaggle May 19 '12 at 19:50
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Yes for RHEL. (Since Feb 22, 2012)

Oracle Offers Free Trial of Innovative Ksplice Zero-Downtime Updates to Red Hat Customers

To continue after the 30 day trial, you need to purchase the Oracle Linux Premier Support package.

Pricing starts at $1,400 for systems with two physical CPUs and $2,300 for systems with more than two physical CPUs. You can configure your systems to automatically install updates as they become available.

It had been offered for both RHEL and CentOS (as well as other operating systems) prior to July, 2011 when Oracle acquired Ksplice. Prior customers are still supported.
Oracle buys Ksplice

The Ksplice code was created and distributed under the GPL v2 and other open source licenses. After Oracle acquired Ksplice and limited availability to Oracle Linux, Ubuntu Desktop, and Fedora, there was discussion about maintaining a CentOS fork of Ksplice:
Ksplice and CentOS

It was also argued that Ksplice was violating the GPL:
Ksplice currently violates the GPL

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