Thanks. If run as an unprivileged user, the dd command will not consume ALL of the disk space (so privileged processes will not be disrupted). It will consume up to the free space limit (default 5%) as described here: http://blog.serverbuddies.com/using-tune2fs-to-free-up-disk-...

The zerofree command looks useful, but I don't know how portable it is. The dd method works across many platforms (such as AIX).