[CentOS] What happened if install a el7 package on a el6 system

[Peter]

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