You're probably referring to just a
/tmp folder under the
/ partition (seen a lot). But it your case,
/tmp is not just a directory under the
/ partition, it's a seperate mount point (just like
/ is the root mountpoint).
In other words, the
/tmp you see is not a sub-directory of
/, it's a seperate partition on your hard disk that can just be accessed through this path. You can see it as a sort of shortcut to that part on your harddisk. Filling up the
/tmp will not affect the disk space usage of your
If you run
cat /etc/fstab (which is the filesystem table file, holding all of your partition data) you can check out what your partitions are configured like and you should see
/tmp as a seperate line.