Matt Dillon responded to a question from David Cuthbert about partition letters; as part of that, he recommended this sort of partion layout:
If you have a large system, it is often a good idea to separate out oft-written directories such as
/usr/obj, and to make/tmplarger./var/tmpis usually made a softlink to/tmp. If you have or intend to process a lot of mail, making/varlarger is a good idea. If you are running a mail server it is often a good idea to make/var/spoolits own partition (and/var/mailits own partition if you are running a large mail pop service or have a lot of users). If you are running a large web server making/usr/local/wwwits own partition (the base of Apache’s site directory) is a good idea.
