In my entry on how we have lots of local customizations I mentioned that the amount of customization we do to any particular Ubuntu server depends on what class or type of machine they are. That's a little abstract, so let's talk about how our various machines are split up by type.

Our general install framework has two pivotal questions that categorize machines. The first question is what degree of NFS mounting the machine will do, with the choices being all of the NFS filesystems from our fileservers (more or less), NFS mounting just our central administrative filesystem either with our full set of accounts or with just staff accounts, rsync'ing that central administrative filesystem (which implies only staff accounts), or being a completely isolated machine that doesn't have even the central administrative filesystem.

Servers that people will use have to have all of our NFS filesystems mounted, as do things like our Samba and IMAP servers. Our fileservers don't cross-mount NFS filesystems from each other, but they do need a replicated copy of our central administrative filesystem and they have to have our full collection of logins and groups for NFS reasons. Many of our more stand-alone, special purpose servers only need our central administrative filesystem, and will either NFS mount it or rsync it depending on how fast we want updates to propagate. For example, our local DNS resolvers don't particularly need fast updates, but our external mail gateway needs to be up to date on what email addresses exist, which is propagated through our central administrative filesystem.

On machines that have all of our NFS mounts, we have a further type choice; we can install them either as a general login server (called an 'apps' server for historical reasons), as a 'comps' compute server (which includes our SLURM nodes), or only install a smaller 'base' set of packages on them (which is not all that small; we used to try to have a 'core' package set and a larger 'base' package set but over time we found we never installed machines with only the 'core' set). These days the only difference between general login servers and compute servers is some system settings, but in the past they used to have somewhat different package sets.

The general login servers and compute servers are mostly not further customized (there are a few exceptions, and SLURM nodes need a bit of additional setup). Almost all machines that get only the base package set are further customized with additional packages and specific configuration for their purpose, because the base package set by itself doesn't make the machine do anything much or be particularly useful. These further customizations mostly aren't scripted (or otherwise automated) for various reasons. The one big exception is installing our NFS fileservers, which we decided was both large enough and we had enough of that we wanted to script it so that everything came out the same.

As a practical matter, the choice between NFS mounting our central administrative filesystem (with only staff accounts) and rsync'ing it makes almost no difference to the resulting install. We tend to think of the two types of servers it creates as almost equivalent and mostly lump them together. So as far as operating our machines goes, we mostly have 'all NFS mounts' machines and 'only the administrative filesystem' machines, with a few rare machines that don't have anything (and our NFS fileservers, which are special in their own way).

(In the modern Linux world of systemd, much of our customizations aren't Ubuntu specific, or even specific to Debian and derived systems that use apt-get. We could probably switch to Debian relatively easily with only modest changes, and to an RPM based distribution with more work.)