If a computer is not equipped with the hardware required to run a specific version of VMware ESX or ESXi, a number of errors can occur during or after the installation process. If a machine lacks a compatible BIOS or CPU -- equipment central to the operation of the system, in other words -- setup will usually fail to initialize or complete. Sometimes, however, the hardware configuration is just enough to get the hypervisor up and running -- and that's where most of the problems occur. Incompatibilities can cause hardware to stop operating or fail to respond, produce error messages when booting a host or virtual machine, and result in poor performance when running VMware vCenter Server or accessing a datastore or virtual network.

Before installing or upgrading to a version of ESX or ESXi, it's important to confirm that the hardware of the target system is compatible with the hypervisor. Even if a previous version of ESX/ESXi works with the machine, that doesn't mean a more recent version will work just as well. Review the minimum requirements to confirm that your hardware is supported.

Some hardware issues can be worked around, however, and some errors that seem hardware-related are instead caused simply by misconfigured settings. Below is a list of common compatibility errors or issues you might encounter after installing the hypervisor, configuring resources, or creating a virtual machine, as well as solutions for resolving these problems.

Datastores

ESX/ESXi 4.0 does not support LUNs larger than 2TB in size. If the size of a LUN surpasses 2TB, the host will fail to assign block devices, or storage resources, to the LUN, and prevent VMware from creating a datastore on the device.

You can use extents to work around this limitation. An extent is a partition on a LUN. A datastore can consist of up to 32 extents. Since each LUN is limited to 2TB in size, and a datastore can utilize 32 extents in total, volumes of up to 64TB in size are possible in VMware. In other words, if you need a datastore to be larger than 2TBs, spread the datastore across multiple LUNs to expand the size of the volume.

To expand a datastore in VMware:

1. Select the target host. Click "Configuration" and then select "Storage" from the Hardware pane.

2. Right-click the target datastore and then click "Properties." Click "Increase."

3. Select the appropriate LUN from the options and then click "Next." To create a new extent, select a LUN marked as un-expandable (listed as "No" under the "Expandable" category); to increase the size of an extent, select a LUN marked as expandable. Select the appropriate disk layout and then click "Next."

4. Check "Maximize Capacity" to use all available disk space, or enter the desired amount in the applicable field. Click "Next."

5. Review your options on the summary screen and then click "Finish."

Central Processing Unit

While older versions of the hypervisor, such as ESX/ESXi 3.5, can run without issue on 32-bit processors, ESX/ESXi 4.0 and ESXi 5.0 both require 64-bit CPUs. ESXi 5.0 has additional hardware requirements above what ESX 4.0 and ESXi 4.0 require, as well: the latest version of the hypervisor must run off, at minimum, a dual-core CPU that supports SAHF and LAHF (instructions that control which flags are loaded into the upper half of the AX register, the latter of which is used to gather the results from arithmetic and logic calculations). Chip manufacturers didn't begin integrating SAHF and LAHF into the 64-bit instruction set until 2005, however, so even if you're running a 64-bit multi-core CPU, it still might not work with ESXi 5.0. If the CPU isn't compatible with the hypervisor, installing/upgrading ESX/ESXi will fail.

Even if the host is able to run off the processor, however, certain virtual machines might not. As to be expected, a 64-bit guest operating system utilizes a 64-bit CPU. However, just having a processor with a 64-bit instruction set isn't enough to run a 64-bit virtual machine; the CPU also must have virtualization technology enabled.

If VT is disabled, the following error is likely to appear when attempting to run a 64-bit guest OS in VMware: "Host CPU is incompatible with the virtual machine's requirements at CPUID level." Enabling Intel VT-x or AMD-V in the BIOS should resolve the above error, allowing the virtual machine to boot. Since the BIOS varies based on the make and model of the motherboard or computer, consult the documentation for your machine to determine how to enable virtualization technology.

Optical Drive

When performing a cold migration, the error "Incompatible device backing specified for device" will appear if the optical disc drive is misconfigured. To reconfigure the drive in vSphere:

1. Right-click the virtual machine and then click "Edit Settings."

2. Select the CD- or DVD-ROM from the Hardware tab. Choose "Client Device" as the device type and then click "OK."

If the above steps fail to resolve the error, check the VMX file associated with the virtual machine to see if the optical device is connected in the configuration data. If the file indicates that the optical device is connected, remove and then re-add the CD- or DVD-ROM from the same window.

TEAC DV-28 series DVD-ROM drives can also cause, at random, an ESX or ESXi 4.0 host to stop responding in vCenter Server or refuse connections from VMware vSphere. While certain models are incompatible with ESX/ESXi altogether, upgrading the outdated firmware on some devices will resolve the issue and restore function to the host. (Some manufacturers, such as Dell, provide firmware flash upgrades for certain TEAC DV-28 series models.)

Virtual Disks

When attempting to run a virtual machine created in another VMware virtualization platform, such as VMware Workstation, the VM will fail to start and the message "Unsupported and/or invalid disk type" will appear. This error occurs because the virtual disk formats associated with the different virtualization platforms are incompatible with ESX/ESXi.

You can use VMware vCenter Converter to convert the disk and import the virtual machine into vSphere. This is a called a cold migration in ESXi (V2V).

Running the following command from the server console will also convert the virtual disk to a format that ESX/ESXi can understand:

vmkfstools -i <original file>.vmdk <new file>.vmdk

Networking

VMware vSphere can run virtual standard switches (vSwitches), which apply to a single host, or virtual distributed switches (vDS), which span across multiple hosts. If the license is incompatible with the vDS feature, vCenter Server will fail when creating a distributed switch. To work around this error in vSphere 5, you must upgrade each host to the Enterprise Plus license.

Conclusions

It's important to keep in mind although there are workarounds for some compatibility issues, if a critical hardware device isn't supported by the hypervisor, there's little recourse. For example, if the hypervisor requires a 64-bit CPU but your system only includes a 32-bit processor, you have no choice but to upgrade the hardware or stick with an older version of ESX/ESXi.