Summary: How to share RDM disks between VMs
Date: Around 2012
Refactor: 20 February 2025: Checked links and formatting.

This was something that took me a few minutes to figure out. I had a few RDM disks that served as shared storage, but after moving the VMs to different storage I had to reconnect them to both VMs. And that's when the problem occurred. After assigning the RDM disk to one VM, I couldn't connect it to the other one. I didn't see it in the list of available storage.

The solution is to add them as an existing disk. When you first add the LUN to a VM as a RDM, a RDM mapping file was created. Once this is done, you will have to add the same RDM to other VMs by adding it as an existing virtual disk and pointing to the RDM mapping file.

Note that if you want to share disks between VMs you'll have to configure a separate SCSI controller for these RDMs on which you set the SCSI bus sharing policy so that sharing is possible: