Application modernisation
VHDX
Virtual Hard Disk v2
VHDX, Virtual Hard Disk v2, is Microsoft's virtual hard disk format, originally built for Hyper-V virtual machines. It also became the original container format used for MSIX App Attach on Azure Virtual Desktop and Windows 365, where a VHDX file holding one or more MSIX packages is attached to a user's session at logon, so the application appears available without having been installed into the golden image or session host itself.
Why VHDX matters in a Microsoft estate
VHDX matters during Windows 11, Intune, Azure Virtual Desktop, and Cloud PC programmes because application blockers can delay the whole rollout. The practical question is whether the term helps capture, package, sign, deploy, or troubleshoot an app with less rework.
How VHDX shows up in practice
A VHDX-based App Attach deployment stores an application's MSIX package inside the virtual disk, hosts that disk on a file share reachable by the session hosts, and mounts it as the user signs in. This lets a single application image be shared across many session hosts and updated centrally without touching the underlying image, avoiding the operational overhead of rebuilding and redeploying a full golden image every time an application needs to be added, updated, or removed. VHDX supports both fixed and dynamically expanding disk allocation.
Because it is a full virtual disk format rather than a lightweight package container, it carries more overhead at mount time and in storage footprint than the newer CimFS-based App Attach approach that has become the preferred method for new deployments, since a CIM image mounts faster and single-instances shared content more efficiently than a VHDX's block-based structure allows. This does not make VHDX obsolete. It remains fully supported, plenty of existing App Attach estates were built on it before CimFS-based delivery matured, and some tooling or scenarios still default to it. Packaging and virtual desktop teams therefore need to be able to work with both formats, rather than assuming every environment they touch has already moved to CIM.
Troubleshooting VHDX-based App Attach in practice tends to concentrate on a specific, recurring set of issues: file share permissions and throughput affecting mount time at logon, and disk expansion behaviour when using dynamically expanding VHDX files under load. Another is the operational discipline of keeping the application versions inside deployed VHDX files current as the underlying MSIX packages are updated, since an outdated VHDX will keep serving an old application version until the disk itself is refreshed. For organisations planning or auditing an AVD or Windows 365 estate, knowing whether App Attach is running on VHDX or CIM is a genuinely practical distinction, not a purely technical footnote, because it affects logon performance, storage costs, and which generation of App Attach tooling and guidance actually applies to that environment.