1.참고 자료

DESCRIPTION : / file system 100% 일 경우 유용한 command.

/tmp: If several large files are here, reboot and they will be
      cleaned out.
/dev: Large files may appear here when trying to write to a device
      andusing the incorrect device name (i.e. /dev/rsto instead
      of /dev/rst0)
 /:    Look for core files
/var: If var is part of the root partition, check for large
      administration files in /var/adm. Also, some spool files may be
      left in the /var/spool directories.

     Check for other large files (over 1mb) with the command:

      find / -size +1000000c -xdev -print

2.결론

tape 작업시 누군가 /dev/rst0 를 주었음. 2.X 에서는 /dev/rst0 가 존재하지않음.
따라서 이것이 약 9 M 정도의 file 이 되어 system full 이됨.