Using tmpfs

Has anyone out there successfully used Sun's tmpfs (puts /tmp in RAM for speed) with Informix? Whenever we try to do anything with it where Informix tries to write to /tmp, it bombs. The system error message is usually "22 Invalid Arguments" and the Informix error varies, but I remember -229 once. Informix says that they haven't heard of this problem before. Oh, we're using 4.11.UC1, FWIW.