Ceph Performance inside Client

Hi,
I have an 3-node ceph cluster with ssd-journal and 10GE-connection.
Inside an VM (wheezy) I got > 100MB/s troughput with dd - but if I cp (or dd) an big file on the same rbd-disk, the performance drop down to 12MB/s.
The networkperformance between host and ceph-nodes is 9.7 GB/s (iperf).

I have tried different cache options, different OS, different filesystems inside VM...

Any hint?

Udo