Are you collecting any performance data? As Barton mentioned, this workload could be taxing the processor, or you could also be gated by the single disk. Perhaps using LVM across multiple disks would help. Understanding whether this is channel, device, or processor limited would help.
I assume the Dell system is also use ext3. If not that would make a difference. Also, the comparison is a little different because of using Ficon instead of FCP SCSI. However, all in all, I have seen other measurements with similar results, particularly for a single guest with a single disk. Bill Bitner - VM Performance Evaluation - IBM Endicott - 607-429-3286
