SDS prefer Partition or Whole Disk for performance

Hi, I have installed a 4 node cluster with 20 SATA disks, however the performance of single threaded read operation and overall write operation is pretty slow. I understood if using cached raid controller will improve write performance. But besides changing hardware, I'm thinking if scaleIO itself aware data alignment on spindle disk? also what is the best practise for SDS,  doing with whole disk or a max-sized partition?

Thanks alot