RAW Device Mapping (RDM)

RDM (Raw Device Mapping)

A Raw Disk Mapping (RDM) can be used to present a LUN directly to a virtual machine from a SAN. Rather than creating a virtual disk (VMDK) on a LUN, which is generally shared with other VMs and virtual disks.

The reasons for doing this should purley be for functional and management reasons, NOT performance. There is a mis-understanding that RDMs offer greater performance compared to VMDK's on a VMFS datastore. I've seen lots of vSphere environments that have gone over kill on RDMs for SQL servers and the like for "performance reasons", its difficult to manage! If your looking for improved storage performance look into the VMware Paravirtual SCSI (PVSCSI) adaptor.

The main reason for using an RDM should be as follows:

To utilize native SAN tools and commands
If using Microsoft Cluster Services (MSCS), Failover Clusters or other clustering solution.

There are two RDM modes to be aware of:

Virtual compatability mode provides vSphere snapshots of this virtual disk.
Physical compatability mode allows the VM to pass SCSI commands direct to the storage system LUN. This allows it to leverage SAN specific features such as interation with the SANs own snapshot functions.

Bringing RDM visible to multiple VM's

First mount the RDM LUN or LUN's to First VM, now on second VM go to edit setting and choose option " Add Existing Hard drive" and browse the RDM pointer drive of First VM and add into it.

Note: If you want to share disks between VMs you'll have to configure a separate SCSI controller for these RDMs on which you set the SCSI bus sharing policy so that sharing is possible:

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