1) The write cache is enabled on the disks.
2) There is something going on with transfers with over a 32kb block size.
3) There is something else that goes on when you add a ZIL besides what is normally known. When I added the separate ZIL device by over 32kb transfer size performance tanked.
I suspect that the slog might be being ignored for writes over 16Kb in size......writing straight to the pool drives with their caches turned on. Can you monitor iostat while running attoto take a look?
What is the log bias attribute set to for this pool / filesystem / zvol ? Latency or Throughput?
Also - might have missed it - what is the block size of the zvol?