On 8/05/19 12:22 AM, Robert Heller wrote:
Many CentOS-7 packages will not install because they will need dependencies
that the EL-6 does not have.
Correct, and different versions of dependencies, and files go in
different locations, etc.
The kernel is different because it is mostly
self-contained and meant to be parallel installed.
Correct.
In most cases, it should
result in an unbootable system because the boot is going to be
dracut+systemd bits and the EL-6 has none of that.
Older versions of dracut will run on newer kernels just fine. When you
install the kernel on CentOS 6 it will run the CentOS 6 version of
dracut at the time of the install and create a CentOS 6 compatible
initramfs image.
Systemd is user-space and does not include components in the kernel (as
far as I'm aware). Even if it does, the kernel is still
backwards-compatible and would boot just fine to upstart (which is the
init system in CentOS 6), it simply would not use those modules and
features that are used for systemd.
And I wonder if the EL7 kernel will even show up as an available kernel. EL7
uses Grub 2 and EL6 uses Grub [1].
CentOS 7 does have grub legacy (1) as an option and does work fine with
grub legacy. I have set up CentOS 7 systems that use grub legacy in the
past. It stands to reason that a kernel that installs and configures
just fine in grub legacy on CentOS 7 will do the same in grub on CentOS 6.
I know that when I installed Ubuntu 18.04 as a *second* OS, that even though
the /boot file system is shared between CentOS 6 and Ubuntu 18.04 the Ubuntu
18.04 installer did not touch /boot/grub/grub.conf and installed
/boot/grub/grub.cfg along side (I manually reinstalled grub 1 and manually
hacked /boot/grub/grub.conf to put the Ubuntu 18.04 boot option in).
This is not the case with CentOS. You can run dual-boot CentOS 6 and 7
on the same grub legacy boot loader and CentOS 7 will boot up and run
just fine.
While I cannot make any guarantees that a CentOS 7 kernel will not cause
issues running in CentOS 6, and indeed I would not support a system that
used such, the Linux kernel, being self-contained and largely
backwards-compatible should in theory, at least, not have issues running
a CentOS 7 kernel on CentOS 6, and indeed there are newer kernels that
are specifically built for CentOS 6 (elrepo kernel-ml) that run just
fine as well.
The main thing that might stand in your way would be any changes to the
rpm file format (which does happen from time to time) that prevent an
rpm built for CentOS 7 from being recognized and installible by rpm or
yum in older versions of CentOS 6. I am aware of such changes from
older versions of CentOS but none between CentOS 6 and 7.
So in summary, it would probably work just fine, but I wouldn't do it,
recommend it or support it.
Peter
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