This is for CentOS 3.1 to 3.8 but the methodology will apply everywhere.
I ran into a problem first of all with a non-booting system after running
#yum update centos-release
It took me to 3.8 and upgraded all the other packages [b:7e931c835d]BUT[/b:7e931c835d]
because of some stupid flags enabled in /etc/yum.conf the KERNEL WASN'T UPGRADED SO AFTER BOOTING, WELL IT DIDN'T BOOT OF COURSE :)
Here's the bad flags:
There was another one something like [code:1:7e931c835d]installonlypkgs=kernel kernel-smp kernel-hugemem kernel-enterprise kernel-debug kernel-unsupported kernel-smp-unsupported kernel-hug[/code:1:7e931c835d]
and it didn't include kernel updates which explains even why "yum update kernel" wouldn't give me the new kernel.
Basically the process to upgrade to a new version is:
yum update yum
yum clean all
yum update centos-release
yum update kernel*
That should be it :)
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