Raw device mapping or RDM for short provides a mechanism for a virtual machine to have direct access to a volume on a FlashArray™. It can be used only with Fibre Channel or iSCSI. RDM can be thought of as providing a symbolic link from a VMFS volume to a physical volume on the Flash Array. It is a mapping file in a separate VMFS volume that acts as a proxy for a raw physical storage device. The RDM contains metadata for managing and redirecting disk access to the physical device. It has several advantages as it enables a one-to-one relationship between a virtual disk in the VM and the volume on the Flash Array. That opens up the possibility of using Flash Array features like snapshots and replication at the virtual disk granularity, and that's a major advantage over VMFS datastores.
RDMs are not fully integrated with the VMware stack. VMware has addressed these limitations with a feature called Virtual Volumes (vVols) that they released in vSphere 6.0. One can think of vVols as the "new and improved" RDM. Because of the advantages that vVols provide over VMFS or RDMs, Everpure recommends that our customers use vVols for Oracle workloads whenever possible. However, due to various constraints, some customers may not be able to move to vVols immediately and for them, RDMs may be the next best option. This document provides detailed instructions on how to create a clone of an Oracle database that is using RDMs.
For instructions on cloning an Oracle database on VMFS, please refer to KB article Cloning an Oracle Database on VMware VMFS.
To learn about cloning an Oracle database on Virtual Volumes, please refer to KB article - Virtualize an Oracle Database on VMware using Virtual Volumes.
RDMs can be configured in two different modes: physical compatibility mode (pRDM) and virtual compatibility mode (vRDM).
Physical mode: Each pRDM is a volume on an array. No I/Os go through the virtual SCSI stack, instead the VM has direct access to the volume with minor exceptions. The OS sees the volume as presented by the array. VM snapshots are not available when RDM is used in physical compatibility mode, but that is not a significant limitation, because with RDMs we would prefer to take FlashArray snapshots, and not VM snapshots.
Virtual mode: Each vRDM is a volume on an array. I/O goes through the virtual SCSI stack, so the guest does not have direct access to the volume and therefore it looks like a VMware "virtual" disk to the OS. VM snapshot functionality is available to RDM used in virtual compatibility mode.
Both physical and virtual RDMs are supported for running Oracle databases. For details on the differences between Physical and Virtual mode, please refer to the VMware document (KB 2009226) - Difference between Physical compatibility RDMs and Virtual compatibility RDMs
Irrespective of the compatibility mode, an RDM disk is a volume on the FlashArray. Therefore the steps for cloning a VM are the same for both modes.