The Pitfalls of Hyper-V Passthrough Disks and Why You Shouldn’t Use Them

Microsoft’s Hyper-V virtualization platform offers various storage options for virtual machines (VMs), including the use of virtual hard disks (VHDs) and virtual machine disk (VHDX) files. However, there is another storage option known as Hyper-V passthrough disks. While they might seem like a straightforward solution, there are significant drawbacks and reasons why experts advise against using passthrough disks in Hyper-V environments. In this article, we’ll explore the concept of passthrough disks and highlight the key reasons you should consider alternative storage strategies.

What are Hyper-V Passthrough Disks?

Passthrough disks, also known as raw device mapping (RDM) or direct-attached storage (DAS), involve presenting a physical disk directly to a virtual machine without using a virtualization layer like VHD or VHDX. Essentially, the VM has direct access to the entire physical disk, bypassing the host’s file system.
Reasons to avoid Hyper-V passthrough disks include:

Limited Portability

One of the primary drawbacks of passthrough disks is their lack of portability. Since VMs are directly tied to specific physical disks, migrating VMs between hosts becomes complex and challenging. This limits flexibility and hampers the ability to scale or reconfigure the virtualized environment easily. One of the main benefits of virtualization is portability and being able to restore VMs anywhere. By using passthrough disks you are effectively giving up this fundamental benefit.

Compromised Backup and Snapshot Capabilities

Passthrough disks make traditional backup and snapshot processes less effective. Virtualization platforms often rely on snapshot technologies to capture the state of a VM at a specific point in time. With passthrough disks, capturing consistent snapshots becomes intricate, potentially compromising data integrity. In fact, passthrough disks cannot be backed up from the host. You would need to back them up from inside the VM. While this works, it’s technically inefficient and difficult to manage in the long-term.

Reduced Integration with Hyper-V Features

Hyper-V offers numerous features, such as live migration, dynamic memory, and checkpoint capabilities, which contribute to the platform’s flexibility and manageability. Passthrough disks do not fully leverage these features, as they operate outside the virtualization layer, limiting the benefits of Hyper-V.

Difficulty in Resource Management

Passthrough disks do not benefit from the dynamic resource allocation and management provided by virtual hard disks. VMs using passthrough disks may experience challenges in optimizing memory usage, and the ability to scale resources dynamically becomes more complex.

Impaired High Availability and Failover Clustering

For environments relying on high availability and failover clustering, passthrough disks present challenges. Migrating VMs between nodes in a cluster can be problematic, potentially leading to downtime and disruptions.

Alternative Strategies

Use Fixed-Size Virtual Hard Disks (VHDs) or VHDX

Opt for fixed-size VHDs or VHDX files for your VMs. These provide better portability, compatibility with Hyper-V features, and simplified management compared to passthrough disks.

Leverage Hyper-V Virtual SANs or Storage Spaces

Hyper-V supports technologies like Storage Spaces and virtual SANs, providing flexible and scalable storage solutions that integrate seamlessly with the virtualization platform.

Explore iSCSI or Fibre Channel Storage

Consider using iSCSI or Fibre Channel storage solutions that allow VMs to interact with shared storage without the drawbacks associated with passthrough disks. This facilitates efficient resource management and ensures compatibility with Hyper-V features.

Summary

While Hyper-V passthrough disks may seem like a straightforward solution for providing VMs direct access to physical storage, the limitations they impose on flexibility, manageability, and integration with key Hyper-V features make them a suboptimal choice. By adopting alternative storage strategies, such as using fixed-size VHDs or VHDX files, exploring virtual SANs, or leveraging iSCSI/Fibre Channel solutions, administrators can build a more agile, manageable, and feature-rich virtualized environment. As virtualization technologies evolve, embracing practices that align with the platform’s capabilities ensures a more robust and future-proof infrastructure.

Additional Hyper-V How-To Guides

Best Practices for Hyper-V System Maintenance

What is Hyper-V? A Simple to Understand Introduction

The Ultimate Hyper-V PowerShell Commands Cheat Sheet

Hyper-V vs VMware: An In-Depth Comparison of Both Platforms

Hyper-V Manager: Everything You Need to Know in 2 Minutes

Unleashing Hyper-V Storage Efficiency: Best Practices for Optimal Performance

Unveiling the Mechanics: Hyper-V Memory Management Strategies

BackupChain: The Best Backup Solution for Hyper-V Virtual Machines and Servers

BackupChain Supports Hyper-V on Windows 11 as well as Windows Server 2022

BackupChain in an all-in-one, reliable backup software for Windows that contains all live Windows server backup features you will ever need, plus a complete virtual machine backup toolset for Hyper-V, VMware, VirtualBox and other hypervisors. BackupChain can be used to create a live VM backup of each VM from the host as a local backup or as cloud backup. Apart from backing up VMs, BackupChain offers a wide range of host-level backup features, as well as Granular File Backup and Granular File Restore to speed up the recovery process.

BackupChain only needs to be installed on the host and each license includes unlimited live virtual machine backup on these main platforms:

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