Recently, we have seen intermittent degradation of I/O performance on the HLRB-II. The exact root cause of this degradation is being investigated. However, there are measures that you as the users of the system can take to optimize your own program's I/O performance. For tuning of I/O, SGI provides a special shared library called FFIO (Fast and Flexible I/O), the use of which is recommended. In particular, FFIO allows to perform unbuffered I/O, which for large files should give much better performance. The FFIO facility requires the following information, which is stored inside environment variables: 1. A pattern for the file names for which FFIO should be used 2. Some parameters for tuning (note that FFIO will need additional memory resources) For running MPI programs together with FFIO a special run script mpiexec_ffio must be used. A more detailed description on how to use FFIO is available at http:// www.lrz.de/services/compute/hlrb/files/ under the heading Efficient use of IO -> FFIO layer. If you encounter any difficulties with using FFIO, or if you see that I /O performance does not improve even with FFIO switched on, please contact the LRZ service desk and provide a detailed description of your problem. This information is also available on our web server http://www.lrz-muenchen.de/services/compute/hlrb/aktuell/ali3965/ Reinhold Bader