The raw disk is a mapping file containing metadata that resides in a VMFS data and is linked with a physical storage device (LUN), allowing a virtual machine to access the storage device directly. This is the right choice if you are planning to allocate a large size volume to some specific application like oracle, or SQL databases.
The component involved with RDM's are:-
In virtual compatibility mode, the RDM file acts as a virtual disk file and In this process, RDM can use a snapshot.
In the physical compatibility mode, the RDM offers direct access to the SCSI device for those applications that require a lower level In general.
In a general scenario, virtual disks are being used however direct access comes into the picture when application required a direct disk (LUN) from a storage system as mentioned above like a database or other application work
VMFS5 and VMFS6 support greater than 2 TB disk size for RDMs in virtual and physical modes.
Steps to follow
Right-click a virtual machine in the inventory and select Edit Settings.
On the Virtual Hardware tab,
Click the Add New Device button and select RDM Disk from the drop-down menu.
The Select Target LUN dialog box opens.
In the Select Target LUN dialog box, select the target LUN for the raw device mapping
Click OK.
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