How to properly remove old kernels RHEL/CentOS 8

With earlier RHEL/CentOS releases we used package-cleanup to remove old kernel packages. But with recent release of RHEL/CentOS 8, package-cleanup cannot be used to remove old kernels. Now we have installonly_limit and oldinstallonly which is used for this purpose. In this tutorial we will analyse this option with different examples.

Previously I had shared a tutorial with best practices to update a kernel with the list of Dos and Donts. Now let me do the same for deleting old kernels from the server.

Best practices to remove old kernels

• Before removing the kernel always check the currently active kernel and the kernel which will get activated post reboot to make sure you don't remove in-correct kernel.
• It is recommended that both currently active kernel and kernel version to be activated post reboot should be same. Although this is not a strict dependency and should be followed case to case basis
• Never use YUM or DNF to remove old kernel packages without specifying the kernel version. We always need at least one kernel package installed on the server while yum/dnf may remove all the installed kernels if you don't specify the version.
• You can use rpm -e <kernel-version> to remove the kernel but you have to make sure you also remove the mapping dependencies. This may be hard for every user to check so you can use yum remove kernel-<version> to remove specific version of the kernel and all it's dependencies
• It is always recommended to keep minimum two kernel installed on the setup for fallback support in production environment.

package-cleanup not working with CentOS/RHEL 8

With the introduction of RHEL/CentOS 8, package-cleanup has been modified and it cannot be used any more to remove old kernels. You can check the difference in the list of supported arguments from their man page in RHEL/CentOS 8 and earlier releases

Now we get this message

# package-cleanup --oldkernels --count=2
package-cleanup has to be executed with one of the options: --dupes, --leaves, --orphans, --problems or --cleandupes

Using installonly_limit with oldinstallonly

Now we can use dnf with installonly_limit and --oldinstallonly to remove old kernel rpms and to define the list of installed kernel rpms. As per the man page of dnf.conf

• installonly_limit: number of installonly packages allowed to be installed concurrently. Defaults to 3. The minimal number of installonly packages is 2. Value 0 or 1 means unlimited number of installonly packages.
• oldinstallonly: Removes old installonly packages, keeping only installonly_limit latest versions.

By default installonly_limit is set to 3 in /etc/dnf/dnf.conf

# grep limit /etc/dnf/dnf.conf
installonly_limit=3

So if you try to install more than 3 kernel packages, the oldest one will be automatically removed.

Removing old kernels with dnf

Example-1: When latest kernel is active

Now let me show you some case study on using dnf to automatically remove old kernel packages. For the sake of demonstration I installed 4 kernels (by modifying installonly_limit in dnf.conf)

At this stage currently my Linux server is active with latest kernel version

# uname -r
4.18.0-193.13.2.el8_2.x86_64

In this command we leave the last 2 kernel installed on the system and remove the remaining old kernels. I have explicitly defined installonly_limit=2 using --setopt to overwrite the default value from /etc/dnf/dnf.conf. If you don't provide this then by default dnf will consider the value of installonly_limit from dnf.conf

# dnf remove --oldinstallonly --setopt installonly_limit=2 kernel

Below is a snippet from my server:

So as you see, this command has initiated the removal of 2 old kernels which is working as expected

Example-2: When older kernel is active

Now in this case, I have switched the active kernel version to older one

# grubby --set-default "/boot/vmlinuz-4.18.0-147.5.1.el8_1.x86_64"
The default is /boot/loader/entries/d88fa2c7ff574ae782ec8c4288de4e85-4.18.0-147.5.1.el8_1.x86_64.conf with index 2 and kernel /boot/vmlinuz-4.18.0-147.5.1.el8_1.x86_64

Next reboot the server and check the active kernel version

# uname -r
4.18.0-147.5.1.el8_1.x86_64

List of available kernels:

So even though I have 193.3.2 release installed, my active kernel is older one i.e. 5.1

Now at this stage if I use the same command to remove older kernels

# dnf remove --oldinstallonly --setopt installonly_limit=2 kernel
Updating Subscription Management repositories.
Error:
 Problem: The operation would result in removing the following protected packages: kernel-core
(try to add '--skip-broken' to skip uninstallable packages or '--nobest' to use not only best candidate packages)

The command fails because it assumes that below kernel versions are latest

kernel-4.18.0-147.8.1.el8_1.x86_64
kernel-4.18.0-193.13.2.el8_2.x86_64

So it tries to remove

kernel-4.18.0-147.5.1.el8_1.x86_64
kernel-4.18.0-147.el8.x86_64

But then kernel-4.18.0-147.5.1.el8_1.x86_64 is the active kernel.

Now I am not sure if this is by design or a BUG. Hopefully it should be by design because the man page clearly says, "Removes old installonly packages" which in our case is the active kernel.

So make sure you check your active kernel before you go ahead and try to remove old kernels

What's Next

I hope you are aware with RHEL/CentOS 8 kernel package contains no files and is there only for backward compatibility. Now instead we have kernel-core, kernel modules to replace the legacy kernel rpm

What is kernel-core and best practices to update them in RHEL/CentOS 8

Conclusion

In this tutorial I shared the best practices with dos and dont's to remove kernels from RHEL/CentOS 8 Linux server. It is important that you are very cautious while removing old kernels or else you may leave your server in broken state. We work with security HF in our production environment which is also responsible for removing kernels, so we rely on LVM snapshots for fallback. So I hope you follow similar practice in your environment.