Why new kernel version cannot be installed on old Ubuntu?

[Valdis Kl=?utf-8?Q?=c4=93?=tnieks]

On Sat, 05 Dec 2020 22:35:37 +0900, Joontaek Oh said:
> So, I am using my laptop with the multi-booting but it is too
> space-consuming.
>
> Moreover, I get work that needs to modify the kernel 3.10.0 for Centos, but
> the kernel 3.10.0 is not installed on the Ubuntu any version, and the
> kernel 3.10.61 or the kernel 4.18.0 is not installed on Centos.

Instead of installing 2 or 3 entire OSs, why not use grub2 for what it's intended for,
and just keep multiple kernels in /boot?

A 1 gigabyte /boot will hold 20 or so copies of vmlinuz- and matching initramfs-,
allowing you to have a 3.10.0, a 3.10.61, a 4.18.0, a 5.8-5.10 kernel or two, and
still have room for a dozen or so builds if you find a need to bisect something.

And the *vast* majority of stuff will Work Just Fine  even if the kernel version
doesn't match the /usr userspace - there's not a lot of programs that actually *use*
the new syscalls we've added since 3.10 or so, and glibc will paper over most of the
mismatches. Of course, *some* stuff needs to be a match, but those usually require
a *precise* uname match - 4.18.9-foo1  and 4.18.9-bar1 aren't matches for that sort of
thing, and the version you need is probably in the tools/ subdirectory of the kernel
source tree matching the running kernel.

If 1 gigabyte is too space consuming, you have *bigger* problems to deal with....
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