RDM is a mapping file and is used to map a LUN directly to VM bypassing VMFS layer. This LUN can be formatted using any File System (NTFS or FAT32) without the need to format it using VMFS and placing VMDK file on top of it.
The mapping file is located with the virtual machine directory.
RDM has two compatibility modes:
- Virtual Mode:Physical Mode
- In this mode all SCSI commands from the VM to LUN will pass through VMFS layer except READ & WRITE. Those will be passed directly to the LUN.
- If you check the device manager of the VM you will see the RDM LUN listed as VMware Virtual Disk SCSI Disk Device. This is exactly the same as VMDK Drive added to the VM. The physical characteristics of the LUN are hidden from the VM.
- The advantage of vMode is that you can still use Distributed Locking Mechanism of VMFS as well as snapshots of RDM LUN drives.
- In this mode all SCSI commands are passed to RDM LUN directly from the VM expect REPORT command virtualized by VMkernal.
- If you check the device manager of the VM you will see the RDM LUN listed based on the physical name/characteristics of the storage array.
- The advantage of the full exposure of the LUN characteristics to the VM can be seen when using SAN Management Tools and MSCS services. Also, RDM LUNs in physical mode can have a size greater than 2TB.
As far as your question about SDM, there is nothing called SDM in VMware, if you mean SVM - Storage VMotion of a VM with RDM. Refer the below link:
VMware Knowledge Base Migrating virtual machines with Raw Device Mappings (RDMs)
Zaheer L,
VCIX-2021
