Booting CentOS 6 with Grub from CentOS 7

I had CentOS 6 installed on an MBR hard disk. I converted the disk to
GPT (with UEFI) and installed CentOS 7 on a new partition.

CentOS 7 itself seems to work properly.

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

The menu entry is:

menuentry 'CentOS release 6.10 (Final) (on /dev/sda7)' --class
gnu-linux --class gnu --class os $menuentry_id_option
'osprober-gnulinux-simple-92dcc576-4e61-41f3-ae42-d06b3aec01ae' {
insmod part_gpt
insmod ext2
set root='hd0,gpt7'
if [ x$feature_platform_search_hint = xy ]; then
search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt7
--hint-efi=hd0,gpt7 --hint-baremetal=ahci0,gpt7
search --no-floppy --fs-uuid --set=root
linux /boot/ ro
root=UUID=92dcc576-4e61-41f3-ae42-d06b3aec01ae rd_NO_LUKS
SYSFONT=latarcyrheb-sun16 rd_NO_LVM rd_NO_DM crashkernel=auto
initrd /boot/

When I try to boot, I get a message that "linux" and "initrd" aren't
valid commands. If I change them to "linuxefi" and initrdefi", I get
this error:

error: kernel too old
error: you need to load the kernel first

The information in the entry seems correct (partition, UUID, etc.).

I've searched on Google quite a bit, but didn't find anything useful.
Are there any suggestions?


Re: Booting CentOS 6 with Grub from CentOS 7

By Stephen John Smoogen at 10/11/2018 - 14:42

On Tue, 9 Oct 2018 at 21:47, Yves Bellefeuille < ... at storm dot ca> wrote:
Was CentOS6 bootable after you converted the disk to GPT? My luck with
getting it to work has not been good as the conversion usually changes
something very important to the underlying OS which needs a reinstall
or a bunch of 'fiddling' in rescue mode before I could get it to work
at all.

The EL6 kernel and initrd probably do not have the tools in them to
deal with a GPT partition. Your 'best' bet is to boot into an EL6
rescue iso, edit the fstab to know it needs to mount a /boot/efi and
make sure the kernel/initrd images in /boot are efi aware and stick
the right stuff into /boot/efi that is needed. [There are several
different people who have posted similar things in 2016 and such. I am
not sure how much work they will take.]

I am not sure if the /boot/efi partition needs to be different from
the one you got for EL7. From various posts on mailing lists and
such... it confuses a lot of tools when they are the same and on other
systems it breaks if they are different.

I don't have much more I can add to this. I would normally just backup
the EL6, blow it away and run EL6 as a virtual machine from then on. I
know this might not work for most people, but it is what I have ended
up doing.

Re: Booting CentOS 6 with Grub from CentOS 7

By m.roth at 10/11/2018 - 15:12

Stephen John Smoogen wrote:
Um, say *what*?! This wasn't a complete rebuild? Next you'll tell us you
buy hardware from Acme, the company that Wile E. Coyote buys hardware.
I will note that we've been using GPT on secondary drives since we started
buying drives > 2TB, and that's been a number of years now....


Re: Booting CentOS 6 with Grub from CentOS 7

By Yves Bellefeuille at 10/11/2018 - 19:42

I didn't use hardware from Acme. I used sgdisk.

Re: (Solved) Booting CentOS 6 with Grub from CentOS 7

By Yves Bellefeuille at 10/10/2018 - 22:50

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,, apparently

The solution was to install the latest kernel-ml kernel from ELRepo,
<a href="" title=""></a> , 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,
<a href="" title=""></a> , which put me on
the right track.

Re: (Solved) Booting CentOS 6 with Grub from CentOS 7

By Peter at 10/11/2018 - 00:36

On 11/10/18 15:50, Yves Bellefeuille wrote:
I think you'll find that in order to use linuxefi and initrdefi you need
a kernel that is new enough to have efi support. If you have an older
kernel (such as that which comes with CentOS 6) then you need to use
linux16 and initrd16.