As the previous article indicated that tmp can be rather limited, there’s another follow up – why is tmp so big as df indicates, but a du does not show anything? Well, that is most likely a file not yet fully deleted where an application still holds the file descriptor.
in my case, it was a temporary flash file created by chromium, and not being capable to fully unlink that.
the key: lsof
$ lsof /tmp COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME bash 3178 dnsmichi cwd DIR 0,18 300 227 /tmp amarok 3318 dnsmichi 56u REG 0,18 5574 16470 /tmp/kde-dnsmichi/amarokXF3318.css chromium 3596 dnsmichi 23w REG 0,18 652292096 14611 /tmp/FlashXXVsNB8j (deleted) icedove-b 5167 dnsmichi 28u REG 0,18 0 342940 /tmp/nsemail.eml
killing chromium removed the file and resolved the issue.