Performancegraph Storage-IO shown incorrectly in XenCenter

If you can see:

It totally shows nonsense and jumps between 0 and 1 mio IOPS.
If I monitor storage via iostat, it shows values between 0 and - if sequential IO happens, maybe up to 2 - 4.000, but usually it stucks at about 800 IOPS.

I don't have a clue why XenCenter shows that nonsense, but it's already for a while like that, probably XAPI is delivering some weird data or there is a timing problem at data collection.

Seems to work on Creedence TP3 again, but there is only an S-ATA HDD connected, where on the problematic server are 2 RAID-Controllers with cache attached.
I thought that to be a problem, cuz the onboard cache will do high IOPS, but then iostat should show the same results - and it doesn't.