I continue to be amazed everyday at all the awesome features and challenges being tackled by our VMware Engineering organization and yesterday was another example of that. There was a question that was posed internally about emulating an SSD device for a Nested ESXi environment running in VMware Fusion. I figure this would be an easy answer and pointed the user to a blog article I had written a few years ago on how to fake an SSD device in ESXi using SATP claim rules via ESXCLI. It turns out, one of the engineers knew of a better way of emulating an SSD Virtual Disk that can be consumed beyond just Nested ESXi VMs but also for any other guestOSes that supports SSD devices.
So why would you want to emulate an SSD device? Well for a vSphere environment, you may want to try out the new Swap to Host Cache feature from a functional perspective to see how it would work. You might be developing a script to enable this feature and having a "fake" SSD device would allow you to create such a script and test it. For other guestOSes, maybe you want to see how the system would react to an SSD device, perhaps drivers or configurations maybe needed and you would like to run through those processes before installing a real SSD device.
So the solution is actually quite simple and it is just an advanced setting in the Virtual Machine's configuration file (VMX) which can also be appended to using either the vSphere Web Client, vSphere C# Client or the vSphere API. This setting is only supported on Virtual Machines that is running virtual hardware 8 or greater. To configure a specific virtual disk to appear as an SSD, you just need to add the following:
scsiX:Y.virtualSSD = 1
where X is the controller ID and the Y is the disk ID of the Virtual Disk.
This configuration presents to the guestOS the mediumRotationRate field of the SCSI inquiry pages 0xB1 and sets the value to 1 and the guests will then report it as a solid-state device. As you can see, this can benefit more than just running Nested ESXi, you can also do various testing on other guestOSes that you require a "fake" SSD device.
Note: Though you can emulate an SSD device, it is no substitute for an actual SSD device and any development or performance tests done in a simulated environment should also be vetted n a real SSD device, especially when it comes to performance.
It is also important to note that reporting of an SSD device will highly depend on the guestOS, here is a high level table on how some of the common guestOSes recognize SSD devices.
| GuestOS | SSD Reporting |
| Windows 8 | IDE, SCSI and SATA disks can be recognized as SSDs |
| Windows 7 | IDE and SATA disks can be recognized SSD, but SCSI as mechanical |
| Linux (Ubuntu & RHEL) | IDE, SCSI and SATA disks can be recognized as SSDs |
| Mac OS X | SATA can be recognized as SSDs, but IDE and SCSI as mechnical |
Here is a screenshot of a Nested ESXi host with an emulated SSD device:
Here is a screenshot of the new Windows 8.1 Preview with an emulated SSD device:
Note: Though I demonstrated this using vSphere, this also works for VMware Fusion (tested this personally), Workstation and Player. The only requirement is that you are running virtual hardware 8 or greater and that your guestOS supports reporting SSD device.
From a Nested ESXi perspective, I will definitely be using this method instead of using ESXCLI to go through the SATP claim rules, this is much easier to remember. I would also like to thank Regis Duchesne for sharing this tip and Srinivas Singavarapu and the virtual devices team for developing this awesome feature. You guys ROCK!
