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wpwood3 24th January 2008 22:01

Safely removing unneeded Linux kernels
This mini howto is specifically for CentOS 5.x and RHEL 5.x but other OS'es have a similar ability.

Your boot partition is getting full of old unused Linux kernels from past kernel updates.
To see what percentage of your /boot partition is currently in use run the "df -h" command:

[root@centos ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda4            123G  19G  98G  16% /
/dev/sda1              99M  21M  73M  23% /boot
tmpfs                2.0G    0  2.0G  0% /dev/shm
/dev/sda3            2.0G  43M  1.8G  3% /tmp

My /boot is at 23% which is fine.

The safe way to manually remove unneeded kernels is by using yum.
First use "yum list" to find out what kernels are installed:

yum list kernel.*

Yum will run and give you something like this:

kernel.x86_64                            2.6.18-8.1.15.el5      installed     
kernel.x86_64                            2.6.18-53.1.6.el5      installed     
kernel.x86_64                            2.6.18-53.1.4.el5      installed

As you can see, I currently have 3 kernels installed. It's good to keep 3 to 5 kernels just in case a kernel update breaks something and you have to drop back to an older version.

But, if I wanted to remove the oldest kernel, here's what to do...

The oldest kernel is the one with the lowest number. So in the example above that would be 2.6.18-8.1.15.el5. The newest kernel and the one I am currently using is 2.6.18-53.1.6.el5. Note that the order yum displays the kernels is NOT in order of oldest to newest! You have to look at each number.

Ok. To remove the oldest kernel in this example you would issue this command:

yum remove kernel-2.6.18-8.1.15.el5

Yum will run as it normally does and ask for y/N final approval. When you type "y" and enter, yum removes the old kernel and updates your grub.conf to remove any references to the old kernel.

Fortunately, there is an automatic way to keep a fixed number of the latest kernels in /boot. There is a yum plugin called "installonlyn". Take a look at /etc/yum/pluginconf.d/installonlyn.conf

# this sets the number of package versions which are kept

As you can see, I have enabled this plugin with "enabled=1" and have set the total number of kernels I want to keep to 3 by setting "tokeep=3". As I said earlier it's good to always keep 3 to 5 kernels...just in case.

By enabling installonlyn as shown above, you will not have to manually remove kernels again. The next time you do a kernel update, yum will automatically remove the oldest kernel to keep you at the "tokeep" number you have set. NOTE- installonlyn will not remove your current working kernel so you don't have to worry about that. If you don't want any kernels to be removed you can set "tokeep=0".

peterpallesen 18th December 2011 12:30

I kinda have the opposite problem. When I install a new kernel the old one is automatically deleted. Tried everything to have yum leave the old kernel behind but it just don't happen. What am I doing wrong?

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