[CentOS] (Solved) Booting CentOS 6 with Grub from CentOS 7

Thu Oct 11 02:50:02 UTC 2018
Yves Bellefeuille <yan at storm.ca>

I myself wrote:

> Grub from CentOS 7 automatically created an entry for CentOS 6, but
> I get an error if I try to boot CentOS 6.

It turns out that Grub2 checks the version of the kernel and refuses
to proceed if it thinks that the kernel is too old. I don't know how
old is considered too old, but the latest centosplus kernel for CentOS
6, vmlinuz-2.6.32-754.3.5.el6.centos.plus.x86_64, apparently

The solution was to install the latest kernel-ml kernel from ELRepo,
https://elrepo.org/ , to change grub.cfg to reflect the new versions,
and to change linux and initrd to linuxefi and initrdefi.

I'm surprised this has apparently never arisen before. Am I really the
first person to dual-boot CentOS 6 and 7?

Thanks to Lingzhu Xiang for his bug report,
https://bugzilla.redhat.com/show_bug.cgi?id=894300 , which put me on
the right track.

Yves Bellefeuille
<yan at storm.ca>