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Created Oct 17, 2019
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How to passthrough SATA drives directly on VMWare ESXI 6.5 as RDMs

How to passthrough SATA drives directly on VMWare EXSI 6.5 as RDMs

There aren't many tutorials about this, the only tutorials I've found were about passing through entire PCIe cards to VMs, or refered to old ESXI versions (below 6.5) that used a more comprehensive desktop client instead of the web app. In v6.5, the web app was introduced and the desktop client was deprecated. You used to be able to setup RDMs in the desktop client, but with the introduction of the web console, this is no longer the case. This tutorial shows you how to pass SATA HDDs to the virtual machine on VMWare ESXI 6.5. This tutorial is partially based on VMWare's own KB and the now deprecated Forza IT blog post.

A word about VMWare ESXI 6.7

There is now an option while editing your VM's settings to add a New raw disk when you click Add hard disk. I have not personally tried it yet, but the comments below have confirmed it works.

Summary

We attach the SATA HDDs as a RDM (Raw Device Mapper) into an existing virtual disk in the command line, then on the web app, we attach a new SCSI controller to the VM, and attach the newly created RDM as an "existing HDD" to the newly created SCSI controller.

Prerequisite knowledge

If you're new to EXSI, you should know a few things:

  • A Datastore is basically this separate layer that sits between a physical device/disk and a virtual disk. A datastore can max the storage of a physical disk, or not. A datastore can have many virtual disks. Virtual disks are what a virtual machine will stores files on. In other words Physical Disk > Datastore > Virtual Disks > Your filesystem e.g. EXT4
  • RDM aka Raw Device Mapper is a pointer file that acts like a virtual disk but instead maps directly to a physical disk. RDMs on EXSI 6.5 have to be created in the command line.

Instructions

  1. Before you get started, make sure you've got a VM setup, and running. In my example, I have a virtual machine and virtual disk both conveniently named Ubuntu Storage

  2. In EXSI's web interface, log in, go to the home page. Click on Actions > Services > Enable Secure Shell (SSH).

  3. Open a SSH session to the ESXi/ESX host.

# ssh root@YOUR_EXSI_IP -p 22
  1. Run this command to list the disks that are attached to the ESXi host:
# ls -l /vmfs/devices/disks

It should look like this. We care about the physical disks with the prefix t10. As mentioned in the comments, it may not necessarily start with t10. You can determined the name/path by going to the web console then Storage, then Device, click on the device you're setting up as RDM, and then copying the path.

[root@localhost:~] ls -l /vmfs/devices/disks
total 12221184373
-rw-------    1 root     root     5000981078016 Feb  1 10:04 t10.ATA_____HGST_HDN726050ALE610____________________NAG4P4YX____________

Here I have a HGST 5TB disk attached to SATA that I'm trying to passthrough to my VM. There will be other listings here of datastores, and virtual disks that is not relevant.

  1. Attach a physical disk as a RDM
# vmkfstools -z /vmfs/devices/disks/t10.ATA_____HGST_HDN726050ALE610____________________NAG4P4YX____________ "/vmfs/volumes/Samsung 850 Pro/Ubuntu Storage/HGST_RDM_1.vmdk"
  1. Attach the newly created RDM to the VM
  • Go back to the web app, go to the Virtual Machine, and click Edit.
  • Under Add other device, click SCSI controller (If you already have an existing SCSI controller for RDMs, you don't need to create another one). In other words, you can reuse the controllers.
  • Under Add hard disk, click Existing hard disk and select the newly created vmdk file. You'll have to use the Datastore file browser to select the HGST_RDM_1.vmdk file
  • Once the HDD is created, under the new disk e.g. Hard disk 2, expand it and make sure it's using the newly created SCSI controller. You'll have to click on the drop down and select e.g. SCSI controller 1 and SCSI (1:0)
  • Change Disk Mode to Independent - persistent. If the Disk Mode is greyed out then change Disk compatibility from Physical to Virtual.

Note: We use independent persistent mode because RDMs won't work with VMWare disk snapshots. More thorough explanation here.

FAQ

  1. What if I want to change the RDM to another VM?

Simply rm the vmdk file and go through steps 5 and 6 again with the new virtual disk for the new VM.

  1. How can I passthrough multiple disks to the same VM?

Simply follow steps 4 to 6 but with the new disk. You can mount the RDM drives to the same SCSI controller. In other words, the first drive could be on 0:0, and the next drive could be on 0:1. There is a limit of 4 SCSI controllers per VM. nb: Thanks to commenters for pointing out that you don't actually need a separate SCSI controllers per RDM. Personally, I'd have a separate SCSI controller just for RDMs only for cleaniness.

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