When an RDM is created, it creates a mapping file with the extension
-rdmp.vmdk with size equal to the size of RDM disk/LUN. When SDRS calculates the committed disk space for all the VMDKs present on the datastore, it treats this mapping file also as a virtual disk for the VM. However, this mapping file size will not be calculated while calculating the datastore disk space usage due to which the UI does not show RDM disk size consumption.
SDRS takes into consideration the size of RDM during load-balancing as if it were to move the disk to another datastore, the -flat disk will be of size of RDM. Therefore, if the ratio of demand/capacity is going beyond 1 as per the preceding calculation, SDRS recommends and forcefully displays the best suitable datastore for creating a new virtual machine.