Vmware why use rdm

The file gives you some of the advantages of direct access to a physical device while keeping some advantages of a virtual disk in VMFS. It increases the manageability of files. The RDM disk file has the same. We can understand RDM functionality with below picture. Virtual machine can easily access it, perform reads and writes operations. There are two compatibility modes for a Raw Device Mapping, physical and virtual.

Which option you choose relies on what features are required within your environment. This means that all SCSI commands with the exception of one are passed directly to the Guest OS thus allowing the VM to take advantage of some of the lower level storage functions that may be required.

An RDM used in virtual compatibility mode, also known as a vRDM, virtualizes the physical properties of the underlying storage and as a result appears the same way a virtual disk file in a VMFS volume would appear. In order to determine which compatibility mode should be used within your environment it is recommend you review the Difference between Physical compatibility RDMs and Virtual compatibility RDMs from VMware. Due to the various configurations that are required for each RDM mode, Pure Storage does not have a best practice for which to use.

Both are equally supported. The process of presenting a volume to a cluster of ESXi hosts to be used as a Raw Device Mapping is no different than presenting a volume that will be used as a datastore. The easiest way to accomplish this task is by connecting the volume to a host group on the FlashArray instead of individually to each host. There are times where you will need to determine which volume on the FlashArray is associated with a Raw Device Mapping. Right click on the virtual machine and select "Edit Settings".

Locate the desired RDM you wish to inspect and expand the properties of the disk. Under the disk properties locate the "Physical LUN" section and note the vml identifier. As seen above we are able to confirm that this particular RDM is backed by the volume called "space-reclamation-test" on the FlashArray.

The process for removing a Raw Device Mapping from a virtual machine is a little different than that of removing a virtual machine disk VMDK. Right click the virtual machine and select "Edit Settings". It is important to use the correct mode for the desired functionality. The benefit of each storage access method is dependent on the workload in the virtual machine.

It is easy to get started with one way of deploying storage without considering the long-term implications because of the ease of use of the virtual environment.

For applications with little to no storage need, using raw device mapping is overkill and not recommended.