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I got a warning of my /boot partition is almost full(85%). What should I do? Can I remove one of the backup kernel? How to do it safely?

My partition right now

Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda2 10321208 719856 9077064 8% /
tmpfs 4015460 0 4015460 0% /dev/shm
/dev/sda1 101133 80781 15130 85% /boot
/dev/sda8 253782660 47668764 193222404 20% /home
/dev/sda7 1032088 535840 443820 55% /tmp
/dev/sda3 10321208 4823740 4973180 50% /usr
/dev/sda5 10321208 1807284 7989636 19% /var

The Kernel I have

root@server1 [/boot]# rpm -q kernel
kernel-2.6.32-358.el6.x86_64
kernel-2.6.32-358.18.1.el6.x86_64
kernel-2.6.32-358.23.2.el6.x86_64

The /Boot directory

root@server1 [/boot]# ls -la /boot
total 78741
dr-xr-xr-x. 5 root root 2048 Dec 3 05:33 ./
drwxr-xr-x. 23 root root 4096 Dec 4 05:46 ../
-rw-r--r-- 1 root root 104112 Aug 28 12:43 config-2.6.32-358.18.1.el6.x86_64
-rw-r--r-- 1 root root 104112 Oct 16 14:01 config-2.6.32-358.23.2.el6.x86_64
-rw-r--r--. 1 root root 104081 Feb 21 2013 config-2.6.32-358.el6.x86_64
drwxr-xr-x. 3 root root 1024 Sep 20 20:15 efi/
drwxr-xr-x. 2 root root 1024 Oct 21 15:06 grub/
-rw-r--r-- 1 root root 16191847 Sep 20 20:21 initramfs-2.6.32-358.18.1.el6.x86_64.img
-rw-r--r-- 1 root root 16261655 Oct 21 15:06 initramfs-2.6.32-358.23.2.el6.x86_64.img
-rw-r--r--. 1 root root 16187335 Sep 20 20:16 initramfs-2.6.32-358.el6.x86_64.img
-rw------- 1 root root 3698835 Sep 20 20:27 initrd-2.6.32-358.18.1.el6.x86_64kdump.img
-rw------- 1 root root 3983771 Dec 3 05:33 initrd-2.6.32-358.23.2.el6.x86_64kdump.img
-rw------- 1 root root 3695290 Sep 20 20:21 initrd-2.6.32-358.el6.x86_64kdump.img
drwx------. 2 root root 12288 Sep 20 20:13 lost+found/
-rw-r--r-- 1 root root 185949 Aug 28 12:44 symvers-2.6.32-358.18.1.el6.x86_64.gz
-rw-r--r-- 1 root root 185978 Oct 16 14:02 symvers-2.6.32-358.23.2.el6.x86_64.gz
-rw-r--r--. 1 root root 185734 Feb 21 2013 symvers-2.6.32-358.el6.x86_64.gz
-rw-r--r-- 1 root root 2408641 Aug 28 12:43 System.map-2.6.32-358.18.1.el6.x86_64
-rw-r--r-- 1 root root 2408974 Oct 16 14:01 System.map-2.6.32-358.23.2.el6.x86_64
-rw-r--r--. 1 root root 2407466 Feb 21 2013 System.map-2.6.32-358.el6.x86_64
-rwxr-xr-x 1 root root 4046224 Aug 28 12:43 vmlinuz-2.6.32-358.18.1.el6.x86_64*
-rw-r--r-- 1 root root 171 Aug 28 12:43 .vmlinuz-2.6.32-358.18.1.el6.x86_64.hmac
-rwxr-xr-x 1 root root 4047152 Oct 16 14:01 vmlinuz-2.6.32-358.23.2.el6.x86_64*
-rw-r--r-- 1 root root 171 Oct 16 14:01 .vmlinuz-2.6.32-358.23.2.el6.x86_64.hmac
-rwxr-xr-x. 1 root root 4043888 Feb 21 2013 vmlinuz-2.6.32-358.el6.x86_64*
-rw-r--r--. 1 root root 166 Feb 21 2013 .vmlinuz-2.6.32-358.el6.x86_64.hmac

The Kernel I'm using

root@server1 [/boot]# uname -a
Linux server1 2.6.32-358.23.2.el6.x86_64 #1 SMP Wed Oct 16 18:37:12 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

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.

What should I do?

if you do a uname -a that will report your currently running version.

Per your posting I assume that is 2.6.32-358.23.2.el6.x86_64 is your current running version, so move all the old ones to some other partition where there is adequate space to save, do something like:

mkdir /root/oldkernels
mv /boot/initramfs-2.6.32-358.18.1.el6.x86_64.img /root/oldkernels

The /boot/efi/EFI/centos/grub.cfg file you want to check and it will be easy enough to read the menu code in it, the top one will be the default one you see when booting and also look for the rescue one; you will likely have numerous ones listed. It is here you can also verify what version you are actually running.

I typically just keep the latest one (at the top) and the rescue (at the bottom) in grub.cfg. Know the real grub.cfg (in your case because I see the efi folder) is in /boot/efi/EFI/centos/grub.cfg. You do not edit this file directly, but I would look at this file to verify the files being booted because it is this grub.cfg that is used when booting.

The rescue is typically the kernel version going back to system installation, which can be many versions prior to what you may be running now. For a rescue option, which is probably a good idea in the long run, you simply need to point it to a reliable and working version so the system will at least boot and you can edit files on the disk should a new kernel go belly up after install and not boot or not work. Basically you want at least 2 boot options in the grub menu, your latest one and then some reliable version to fall back on.

you edit /etc/default/grub.cfg and modify this file; make the the menu how you want simply by commenting out the ones you don't want with a #, then do a grub2-mkconfig -o /boot/efi/EFI/centos/grub.cfg

KDUMP is the problem

And from the initrd-2.6.32-358.18.1.el6.x86_64kdump.img files having kdump in the name, it looks like you have kdump enabled. Unless you use it, you can disable kdump which will help save space. And unless you are debugging system crashes and the like, you do not need the *kdump.img files so you can delete those. I don't use kdump, never have, but it is enabled by default during installation and I suspect by default saves to your /boot folder; which if only 100mb is bad. So either modify kdump to dump elsewhere, or you most likely don't use it so disable kdump.