Symptoms
The /tmp partition is full, however, checking "df" compared to "du" shows a discrepancy in disk usage being reported.
Description
This issue can result when there are deleted temporary files being held open. As an example I have included output showing deleted open MySQL files (command used is "lsof"):
mysqld 14291 32659 mysql 9u REG 8,4 0 16 /tmp/ibQ2O6UK (deleted)
mysqld 14291 32659 mysql 10u REG 8,4 0 18 /tmp/ib6mjpK7 (deleted)
mysqld 14291 32659 mysql 20u REG 8,4 0 19 /tmp/ibTnr5kP (deleted)
mysqld 14291 32659 mysql 808u REG 8,4 8388608 23 /tmp/STF6yotG (deleted)
mysqld 14291 32659 mysql 1980u REG 8,4 9613119488 25 /tmp/STmJ8tNo (deleted)
Workaround
If a service has deleted temporary files open, stopping and restarting the service should free up that space. You would then need to review what might be keeping those open of they are lingering longer than appropriate and whether you have queries that are generating inordinately large temporary tables and exhausting your /tmp space.