A common model creates /, /home and /var partitions as
discussed above. This is simple to install and maintain and
differentiates well enough to avoid adverse effects from
different lifetimes. It fits well into a backup model, too:
Almost noone bothers to backup USENET news spools and only some
files in /var are worth backing up (/var/spool/mail comes to
mind). On the other hand, / changes infrequently and can be
backup up on demand (after configuration changes) and is small
enough to fit on most modern backup media as a full backup
(plan 250 to 500 MB depending on the amount of installed
software). /home contains valuable user data and should be
backuped up daily. Some installations have very large /homes and
must use incremental backups.
Some systems put /tmp onto a seperate partition as well, others
symlink it to /var/tmp to achieve the same effect (note that
this can affect single user mode, where /var will be
unavailable and the system will have no /tmp until you create
one or mount /var manually) or put it onto a RAM disk (Solaris
does this for example). This keeps /tmp out of /, a good idea.
This model is convenient for upgrades or reinstallations as
well: Save your configuration files (or the entire /etc) to
some /home directory, scrap your /, reinstall and fetch the old
configurations from the save directory on /home.