My computer should know that /var/log/squid/access.log.9.gz
is less import that /var/spool/squid and /var/log/squid.access.log.8.gz.

Thus when disk space is tight rather than failing, it should free low priority files (obviously with a limit beyond which it has to fail - so I don't lose my data - but so it can lose transient files of no permanent interest). Windows already does this for files in the Trash can (allegedly).

Similarly my database is more important than the contents of /usr/share/doc.

I appreciate these priorities would have to be configurable, because some folks have a legal obligation to keep old email log files for example. So how priorities are assigned is difficult, on the other hand this is about what it should do, not how to do it. I suspect defaults that mean cache files are a low priority, and that old log files are less important than new one.

Sure disk space is cheap - but cache files and log files are a common pattern. I want to cache/log as much as possible but without undue pain.