A Raw Device Map(RDM) is used to directly present a LUN to a Virtual Machine from a SAN. Instead of provisioning the LUN to ESXi and creating Virtual Disks (VMDK) on the LUN, the LUN is attached directly to the Virtual Machine and is not shared.
RDM is used for two main reasons:
There is a big misunderstanding that using RDM increases the performance of the Application/Virtual Machine compared to the traditional VMDKs on the datastore. It is not true. RDM does not increase the performance and they are used only based on the functionality.
In my lab environment, I have created an iSCSI Target called RDM on my Windows Server.
Edit the iSCSI Adapter of the the ESXi Server.
Enter the IP of the iSCSI Target. In my case, its the IP of my Windows Server.
Rescan all adapters to discover the target.
The target will be discovered but do not add it to the ESXi Server.
Edit Settings the Virtual Machine to which you want to map the LUN.
In the New Device, add RDM Disk.
Now select the required LUN and click OK.
That LUN will be mapped as Raw Device to the Virtual Machine.