This article provides information about expanding a Raw Device Mapping (RDM) that is in use by a virtual machine.

• Physical compatibility mode
  
  Physical compatibility mode RDMs, which are also known as pass-thru RDMs, expose the physical properties of the mapped LUN to the guest operating system. For the guest operating system to recognize the added space to the expanded mapped LUN, perform a rescan from the ESX host, then from the guest operating system. This process does not require rebooting the virtual machine or the ESX host.
  
  Many of the time No changes to the RDM files (.vmdk or metadata pointer) are required to take advantage of the added disk space. But it is advisable to unmap the RDM and Re-Map is to the VM to clear any stale cache. 
   
• Virtual compatibility mode
  
  To safely expand the RDM:
  1. Expand the RDM LUN from the SAN side. Contact your vendor for assistance.
  2. Perform rescan on the ESX and verify the new LUN size is observed.
  3. svMotion the RDM pointer files or recreate the RDM mapping with the following steps to update the mapped disk size:
     • Shut down the Virtual machine
     • Note scsiX:Y position of the RDM in VM Settings and then remove the RDM file from the Virtual Machine and delete from disk ( Navigate to VM Settings > Add > Hard Disk > RDM, select the scsiX:Y position that the RDM was using before and then power on the virtual machine)
     • On the array, increase the size of the LUN that the RDM pointed to
     • Re-add the LUN as an RDM to the Virtual Machine
     • Power on the VM and add the RDM with right SCSI info which you have noted earlier 
  4. Perform a re-scan from the guest operating system 

• diskpart
• Partition magic
• Altaris

The following tools can increase the size of a Linux partition:

• parted
• gparted

Caution: To avoid data loss, closely follow vendor documentation.

Note: If you are using third party Recovery Point management software (EMC and possibly others), it does not allow the proper presentation or rescan the LUN.