Expanding the size of a Raw Device Mapping (RDM)

Products

VMware vCenter Server VMware vSphere ESXi

Issue/Introduction

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

Environment

VMware vSphere ESXi

Resolution

Please note:

Warning: Ensure that there are no snapshots on the disk before attempting this operation. If there are snapshots present, commit them. Otherwise, you may experience data corruption. For more information, see Finding and listing virtual machine snapshots
Snapshots can only be taken with RDMs in Virtual Compatibility Mode. If you do not want to use snapshots, the maximum mapped LUN size in Physical Compatibility mode is 64TB (from ESXi 5.x).
When making changes to LUNs presented to one or more VMware ESXi Hosts, you must perform a Storage Rescan on all ESXi Hosts which are presented that LUN so that the new size is consistent across all ESXi Hosts
If you are using 3rd party Recovery Point Management software ( e.g EMC ), it does not allow the proper presentation or rescan the LUN.
To resolve this issue, temporarily remove the Storage from the Recovery Point utility and re-add it after the rescan
For both Physical and Virtual compatibility mode, you may also have to increase the size of the partition

The procedure to expand the size of the RDM depends on the type:

A.) 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 OS.

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 OS.

This process does not require rebooting the virtual machine or the ESX host.

After the physical compatibility mode LUN size is changed, you might notice that the VMware vSphere Web Client does not display the correct size of an RDM, despite Virtual Machine reload

as described in KB 315898

while the Host LUN device and Guest OS correctly report the new size of the RDM disk after rescanning Storage/Disks.

--> The discrepancy in Size reporting is only of cosmetic nature. There is no functional impact. The only exceptions to this would be:

It could pose issues in regards to the outdated sizing information, if you were to ever convert these VMs using these physical RDMs to virtual RDMs or regular vmdks
During migrations/clones such as reported in vSphere 7 RDM to Shared VMDK Migration.

As a result we do recommend to re-create the RDM Pointer file containing the Size information, by removing and re-adding the RDM disk to the Virtual Machine

by following the steps outlined below.

Via the vSphere Client: Process to correct the size information in the VM Descriptor file and in the VM settings:

1.) Shut down the VM(s) which have the affected RDM Disk attached

2.) For all affected VMs: Open the VM Settings

2.) Document RDM disk settings (multi-writer mode/physical mode type/etc.) along with the RDM SCSI controller settings (SCSI ID)

3.) For all affected VMs: Remove the RDM from all of the connected VMs (DO NOT DELETE THE DISK)

4.) For all affected VMs: Re-add the RDM back using the original SCSI controller and disk configuration that was documented before the disk was removed

5.) Power on all affected VMs by "One-By-One" and verify that the RDM Disk is now showing the correct size via the VM Settings

B.) Virtual compatibility mode

To safely expand the RDM:

1. Expand the RDM LUN from the SAN side. If needed: 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

Altering the Partition table information is done within the Virtual Machine Guest OS.

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

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

diskpart
Partition magic
Altaris

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

parted
gparted