Virtual Disk Types

VHD and VHDX

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VHD is the original virtual disk format introduced in Hyper-V in Windows Server 2008. VHDX is essentially version two of VHD and was introduced with Windows Server 2012. It improves the functionality of the VHD in that the maximum storage allocations per disk increases from 2TB to 64TB, introduces live resizing of the disk, and improves metadata management for tasks such as SSD trimming. VHDs are generally associated with Hyper-V Generation 1 VMs, whereas VHDXs are associated with Generation 2 VMs. Because of these advantages in total size and operational improvements, VHDXs on Generation 2 VMs are highly recommended over VHDs.

An example of a VHDX connected to a virtual machine is presented in the image below.

VHD Sets

Disks that can be shared with multiple VMs are possible with the VHD Set feature of Windows Server. A VHD Set replaced the shared VHDX feature. A VHD Set’s purpose is for in-guest features such as a Failover Cluster Instance that require shared storage to be seen inside multiple VMs. For Microsoft’s documentation on VHD Sets, consult “Create Hyper-V VHD Set files”.

Passthrough Disks

A passthrough disk allows a VM to directly connect to a disk presented to the Hyper-V host or on a FlashArray. From a FlashArray perspective, there are use cases such as some integrations where direct access to disks (such as certain snapshot scenarios) may be needed, but the general best practice for VMs is to use virtual disks.

Passthrough disks should be the exception, not the rule. For a standard passthrough disk, it is not part of the cluster resources and must be taken offline during a Hyper-V cluster role failover.

If you need to configure passthrough disks, it is recommended to instead use FC connectivity through vHBAs. While this is a more complex configuration, the benefits are worth it.

From a performance perspective, there should be no difference from virtual disks and unless necessary, use virtual disks instead of passthrough.