How exactly SIOC is leveraged by SDRS from core technical perspective?
This is more close resource to get answer.
http://www.vmware.com/files/pdf/vmw-vsphr-5-1-stor-drs-uslet-101-web.pdf
This is a excellent blog from frank Denneman for your question.
vSphere 5.1 Storage DRS load balancing and SIOC threshold enhancements - frankdenneman.nl
This is more close resource to get answer.
http://www.vmware.com/files/pdf/vmw-vsphr-5-1-stor-drs-uslet-101-web.pdf