[CentOS] questions about upgrading

[isdtor]

> 2) can I upgrade a minor version to any other greater minor number, or only to the greatest?  E.g. if i have a v5.4 box, can i upgrade to v5.5 or v5.6, or only to v5.7 (currently the latest)?

Yes if you configure the repositories in /etc/yum.repos.d accordingly
(i.e. use the vault).

> 3) if an upgrade delivers a new kernel, am I obligated to run it (i.e. will the old kernel remain installed, or will the upgrade remove the old kernel)?  If not, does the upgrade automatically update grub such that the new kernel becomes the default?  Will the upgraded box function properly without running the new kernel?

Kernels are installed side by side. For the second question, see
/etc/sysconfig/kernel.

In theory you should be able to mix and match any components from the
same CentOS major release without problems, unless any of the release
notes say otherwise. Whether it's a good idea is a different matter.

> 4) aside from when an upgrade includes a new kernel, how can I tell when a reboot is necessary?  I gather that a new glibc may require services to be restarted, but is a reboot strictly necessary?

New glibc requires a reboot. Upgrading other system libraries does
too, but I can't give a definite answer on the how to tell. ldd
/sbin/init indicates the libselinux and libsepol packages are other
candidates, but that's just one example.

> 5) is there a way to apply only the updates that were made for the given minor version you're currently at?  E.g. if I'm at v5.5, how can I only apply updates that were made prior to v5.6's availability?

See the answer to 2).

Other people will give you some flak for these questions as it's
generally recommended to run the latest version.