This work was supported by the Mathematical, Information, and Computational Sciences Division subprogram of the Office of Computational and Technology Research, U.S. Department of Energy, under Contract W-31-109-Eng-38; and by the Scalable I/O Initiative, a multiagency project funded by the Advanced Research Projects Agency (contract number DABT63-94-C-0049), the Department of Energy, the National Aeronautics and Space Administration, and the National Science Foundation.