こんな感じ

# yum update kernel
Loaded plugins: fastestmirror, priorities
Setting up Update Process
Loading mirror speeds from cached hostfile
 * extras: centos.mirror.secureax.com
Resolving Dependencies
--> Running transaction check
---> Package kernel.x86_64 0:2.6.32-504.12.2.el6 will be installed
--> Processing Dependency: kernel-firmware >= 2.6.32-504.12.2.el6 for package: kernel-2.6.32-504.12.2.el6.x86_64
--> Running transaction check
---> Package kernel-firmware.noarch 0:2.6.32-504.8.1.el6 will be updated
---> Package kernel-firmware.noarch 0:2.6.32-504.12.2.el6 will be an update
--> Finished Dependency Resolution

Dependencies Resolved

===================================================================================================================================================================================================================
 Package                                               Arch                                         Version                                                    Repository                                     Size
===================================================================================================================================================================================================================
Installing:
 kernel                                                x86_64                                       2.6.32-504.12.2.el6                                        updates                                        29 M
Updating for dependencies:
 kernel-firmware                                       noarch                                       2.6.32-504.12.2.el6                                        updates                                        14 M

Transaction Summary
===================================================================================================================================================================================================================
Install       1 Package(s)
Upgrade       1 Package(s)

Total size: 44 M
Is this ok [y/N]: y
Downloading Packages:
Running rpm_check_debug
Running Transaction Test


Transaction Check Error:
  installing package kernel-2.6.32-504.12.2.el6.x86_64 needs 2MB on the /boot filesystem

Error Summary
-------------
Disk Requirements:
  At least 2MB more space needed on the /boot filesystem.

なので、dfってみた

# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/pdc_efgdjdaaip1
                       93M   62M   27M  71% /boot

/boot 少な！

なにも考えずにインストールすれば250MBとか500MBくらい割り当てられると思ってた。
なんでこんなサイズに？

パーティションサイズ変更するのは結構面倒なので、インストール段階で確認しておきましょう

っていうおち。

コメントを受けての追記

最新のカーネルを3つだけ残して残りを削除

# package-cleanup --oldkernels --count=3

参照元 yum-utilsを使って/bootの不要なカーネルを削除する方法