boot partition is almost full in CentOS

Do the following to keep just the last 2 kernels on your system, to keep /boot clean

1 - Edit /etc/yum.conf and set the following parameter

installonly_limit=2

This will make your package manager keep just the 2 last kernels on your system(including the one that is running)

2 - Install yum-utils:

yum install yum-utils

3- Make an oldkernel cleanup:

package-cleanup --oldkernels --count=2

Done. This will erase in a good fashion the old kernels, and, keep just the last 2 of them for the next upgrades.

For special cases where you have vmlinuz-0-rescue-* and initramfs-0-rescue-* files using too much disk space, please take a look at this question on U&L:

• Removing the rescue image from /boot on fedora

You can delete old kernels safely by doing the following:

# Install the yum-utils if they aren't installed
yum install yum-utils
# Cleanup old kernels and don't keep more than 2
package-cleanup --oldkernels --count=2

And should you wish, you can limit this always by doing the following in /etc/yum.conf

installonly_limit=2

Kernel images are actually really small:

[root@ditirlns01 ~]# ls -lh /boot/vmlinuz-2.6.18-3*
-rw-r--r-- 1 root root 2.2M May  4  2012 /boot/vmlinuz-2.6.18-308.8.1.el5xen
-rw-r--r-- 1 root root 2.2M Jul 27 01:43 /boot/vmlinuz-2.6.18-348.16.1.el5xen
-rw-r--r-- 1 root root 2.2M Mar 22  2013 /boot/vmlinuz-2.6.18-348.4.1.el5xen

There's more to the kernel package, obviously, but that's the part that's on /boot which is what your concern is.

So with a 100MB /boot partition, deleting a 2-3MB kernel probably isn't going to get you very far.

100MB is actually usually way more than people need. I would do enough du -sh invocations so you can see what's taking up all that space, because you shouldn't even be getting kind of close to using 100MB on that mount point:

[root@ditirlns01 ~]# df -h /boot
Filesystem            Size  Used Avail Use% Mounted on
/dev/xvda1             99M   34M   60M  37% /boot

Which is with three kernels installed:

[root@ditirlns01 ~]# rpm -qa kernel*
kernel-xen-2.6.18-348.16.1.el5
kernel-xen-2.6.18-348.4.1.el5
kernel-headers-2.6.18-348.16.1.el5
kernel-xen-2.6.18-308.8.1.el5
[root@ditirlns01 ~]#

I'm willing to wager that someone put a file on /boot as a temporary move and forgot to move it back off later on.