This was written for an MD3000i but its still accurate for your MD3600i. This will take some time to complete and performance will be impacted for the duration of the defrag because the data is physically being rewritten to different areas of the disk to combine the free space. www.mikebrawley.com/.../dell-md3000i-defragmenting-a-disk-group-merging-free-capacity-within-a-disk-group
On our 3600i we recently deleted a few virtual disks, but the space left from deleting them is spread out in chunks of "Free Capacity Nodes".  I would like to make a new disk, but there is no Free Capacity Node that is the size I need.  But when you add them up, it is more than enough.  How can I recombine the free capacity into a single "Node"? 
MD3600i - Consolidate Free Capacity Nodes?