General :: Root File System Partitioning - Filesystems ?

I've been using *Unix systems for many years now, and I've always been led to believe that its best to partition certain dirs into separate FileSystems, off the main root FS.

For instance, /tmp /var /usr etc

Leaving as little as possible on the main / system.

Its so that you don't fill up the root system be accident, by some user putting in too bigger files in /tmp, for example.

I would presume that filling the / system would not be too good for Linux, as it would not be able to write logs and possibly other things that it needs to.

I believe that if root gets full, then there is something like a 5% amount saved for just 'root' to write to, so that it can do its stuff.

However, eventually, / will become full, and writes will fail.

On top of this, certain scripting tools, such as awk, use the /tmp/ system to store temp files in, and awk wont be able to write to /tmp/ as its full, so awk will fail.

However, I'm being advised that there is no need to put /tmp /var etc onto separate FSs, as there is no problem nowerdays with / filling up. So, /tmp /var /usr are all on the root FS.

I'm talking about large systems, with TBs of data (which is on a separate FS), and with a user populations of around 800-1000 users, and 24/7 system access.