How much memory does your guest have? With 2GB, dbench shouldn't do much I/O, it is CPU bound. See our presentation "Linux on zSeries Performance Update" at linuxvm.org for some dbench charts.
Use DASD statistics to verify that you have no DASD bottleneck. Before running the benchmark issue 'echo set on > /proc/dasd/statistics' and when dbench is finished issue 'echo set off > /proc/dasd/statistics' DASD statistics shows, amongst other things, I/O subsystem queueing time and ESS response time. If I/O subsystem queueing time is the problem, LVM or PAV will help. Klaus Bergmann
