Just to clarify:  Do you have it in Optimized mode, or are you trying to use a redundant configuration?
Dear Concern,   one our valued customer facing issue in R810 server memory utilization from memory redundancy option.   I have installed physical memory 128GB (4GB module) but showing 112GB from OS.   Our Troubleshoot step:   Note: Memory Redundancy Option set always Disable and not changeable like Spare, optimization or mirror.  Please suggest me how can I solve this issue.
Memory Utilization issue in Dell Server R810.