How to add a Raw disk to your esx host (RDM)

RDM, raw device mapping allows for a virtual guest to access a provisioned disk or lun directly and perform operations on it.
Use physical if you are interfacing with a physical server or have software that will perform direct interfacing with the lun.
Use virtual if you want to provide faster storage access than vmfs shared luns yet use other techniques like snapshots.




Mike Deardurff's words::




Mainly though, youll find the difference between virtual and physical though is that virtual mode present thes LUN as a virtual disk allowing it to still utilize VMwares snapshot features while physical mode give direct access to the SAN and in some peoples opinion, a greater level of performance.


Pasted from <http://communities.vmware.com/message/644565>




Have vmware find the lun that you will be using for RDM
Hit RESCAN ALL













Click OK





Now go to your vm and edit it's settings. Click Add









Select hard disk





Raw  Device mappings :select that for this session







Select the LUN that you allocated.





Store with the virtual machine is the best option.







Choose the method of RDM you want for the VM
If you aren't sure and have no physical servers accessing  it, like in a MSCS (Microsoft cluster) , Use virtual







Leave it as default unless you have a reason to make the disks independent from the guest.



 Click Finish











Click ok











Now you will see a new disk in your server. Format it as you please.