Troubleshooting: Virtual Machine LUN Disappearance After MPIO Configuration in Windows Server
Applies to: Windows Server 2022, Windows Server 2019, Windows Server 2016, Microsoft Hyper-V Server 2016
Symptoms¶
You are operating a virtual machine (VM) host environment utilizing Windows Server 2016 or a more recent version of the operating system. As part of your storage configuration strategy for VMs hosted on this platform, you implement a series of steps designed to attach physical storage devices directly to a VM and then configure path redundancy for these devices within the VM. This process typically unfolds as follows, leading to an unexpected outcome with the storage visibility inside the guest operating system.
Initially, you install essential roles and features on the VM host server. This includes enabling the Multipath I/O (MPIO) feature, which provides fault tolerance and performance improvements for connections to storage devices, and the Hyper-V role, which enables the host to run virtual machines. With the host properly configured, you proceed to create a new virtual machine or utilize an existing one on this host server. Inside the newly created or selected VM, you also install the MPIO feature to manage multiple paths to storage devices from the perspective of the guest operating system.
Subsequently, you leverage the Discrete Device Assignment (DDA) capability, a feature that allows physical PCI Express hardware devices on the host to be directly passed through and attached to a virtual machine. You use DDA to connect one or more physical storage LUNs (Logical Unit Numbers), often referred to as pass-through disks, directly into the VM. Upon inspecting the configuration settings of the VM within the Hyper-V Manager interface, you observe that the DDA-assigned disks, representing the physical LUNs, are correctly listed under the VM’s hardware settings, explicitly identified as physical disks attached via DDA.
Connecting to the VM and opening the Disk Management tool within the guest operating system shows the newly attached LUNs. At this stage, these disks are typically listed as unallocated space, indicating that they are recognized by the operating system but have not yet been initialized or partitioned. Following standard MPIO configuration procedures within the VM, you access the MPIO Settings tool in the guest operating system. You configure the DDA-assigned LUNs to be managed by MPIO, expecting the feature to discover and utilize multiple paths to these devices if available. After applying the MPIO settings, a restart of the virtual machine is performed to ensure the configuration changes take effect.
However, upon the VM completing its restart cycle and becoming available again, you connect to the guest operating system to verify the storage configuration. Opening the Disk Management tool once more reveals an unexpected issue: the DDA-assigned LUNs that were visible and listed as unallocated disks before the restart and MPIO configuration are now no longer displayed in the tool. They have effectively disappeared from the VM’s view. This absence persists despite the disks still being listed in the Hyper-V Manager settings for the VM, indicating a conflict or issue arising from the combination of DDA and MPIO within the guest.
Status¶
This observed behavior, where DDA-assigned LUNs configured for MPIO within a virtual machine disappear from the guest’s view after a restart, is considered by design. It is not a defect or bug but rather a limitation inherent in the architectural interaction between the Discrete Device Assignment feature and the Multipath I/O feature when applied in this specific configuration context. Understanding the underlying mechanisms of both features clarifies why this outcome occurs and why it cannot be achieved with this setup.
More Information¶
The root cause of the LUNs disappearing after MPIO configuration within a DDA-assigned VM lies in how both Discrete Device Assignment (DDA) and Multipath I/O (MPIO) fundamentally interact with the hardware path from the host server to the storage device. Specifically, both features rely on, and in essence, require control over the Host Bus Adapter (HBA) or the underlying connection path that links the physical LUN to the computing environment. This creates a conflict when trying to use both simultaneously for the same device in the manner described.
DDA functions by bypassing the Hyper-V virtualization stack for specific PCI devices, including HBAs or storage controllers. When you use DDA to assign a physical LUN to a VM, Hyper-V effectively grants the VM near-direct, exclusive access to the HBA port or controller responsible for connecting to that LUN. This pass-through mechanism is designed to provide low latency and high performance by minimizing software layers between the VM and the hardware. From the perspective of the host, once a device or its path is assigned via DDA, the host operating system relinquishes its direct management and control over that specific hardware resource and its associated paths.
Conversely, Multipath I/O (MPIO) is a feature designed to enhance data path availability and performance by managing multiple physical connections from a server to a storage device. For MPIO to function correctly, either the host operating system or the guest operating system (depending on where MPIO is configured and how storage is presented) must have visibility and control over all available paths to the target LUN. The MPIO software stack aggregates these paths and presents a single, resilient device to the operating system and applications.
Here is a conceptual diagram illustrating the conflict:
```mermaid
graph TD
A[Physical LUN] → B[HBA/Storage Controller]
B → C[Host Server]
C → D[Hyper-V Virtualization Stack]
D → E[Virtual Machine (VM)]
subgraph Attempted Configuration
C -- DDA --> E
B -- part of DDA path --> E
E -- MPIO configured in VM --> F[MPIO Stack in VM]
F --> B
end
G[Host MPIO Stack] --> B
classDef conflict fill:#f9f,stroke:#333,stroke-width:2px;
B:::conflict
E:::conflict
note right of B: HBA/Controller is core conflict point
note left of C: Host MPIO manages paths to LUN
note left of E: VM MPIO attempts to manage paths via DDA
note right of E: DDA gives VM control of HBA/Controller path, bypassing Host MPIO
note left of F: VM MPIO needs multiple paths presented
style G fill:#ccc,stroke:#333,stroke-width:2px
```
When you configure DDA for a LUN, you are essentially giving the VM control over the specific path, potentially including the HBA port, that leads to that LUN from the host. This action removes the LUN and its associated path from the host’s standard storage management, including any MPIO configuration the host might have for that device. The VM then sees the device via the passed-through hardware connection.
When you subsequently install and configure MPIO within the VM for this DDA-assigned LUN, the MPIO software in the guest operating system attempts to discover and manage multiple paths to the LUN. However, because the device was passed through via DDA, the VM’s view of the storage path is typically that of a single, direct connection presented by the underlying hardware controller that DDA exposed. The VM’s MPIO is not receiving information about multiple distinct paths from the host’s storage fabric; it only sees the connection presented via the DDA-assigned device.
The MPIO configuration process, especially after a restart, involves the MPIO driver taking control of eligible devices. In this scenario, the MPIO driver in the guest OS identifies the DDA-assigned LUN and attempts to manage it. However, the underlying passthrough mechanism via DDA does not present the device in a way that is compatible with the VM’s MPIO stack expecting to find and control multiple distinct paths to the same LUN identifier (like a specific SCSI device ID). This mismatch or inability for the guest MPIO to properly enumerate and take ownership of the single, DDA-presented path results in the device not being successfully claimed and presented by the MPIO layer in the guest OS. Consequently, the LUN disappears from the guest’s view in tools like Disk Management because the storage stack requires a successfully initialized driver (in this case, the intended MPIO driver) to present the device, and that process fails or results in the device being hidden.
Therefore, the conflict arises because DDA dedicates the hardware path to the VM, effectively bypassing the host’s storage stack and presenting a specific view of the device to the guest. MPIO in the guest requires a different type of presentation, needing multiple paths to be visible and manageable, which the DDA passthrough does not provide. Both features cannot simultaneously control or manage the same underlying connection path or require conflicting modes of presentation from the host hardware layer. The system is designed such that attempting this configuration leads to the LUN not being surfaced correctly in the guest after MPIO attempts to take control.
While DDA offers benefits like reduced CPU overhead and improved throughput for direct storage access, it is intended for scenarios where the storage device is managed as a single unit within the VM. For configurations requiring MPIO benefits (path redundancy and load balancing) for block storage presented to a VM, alternative methods are typically used. These might include configuring MPIO on the host and presenting storage to the VM via virtual hard disks (VHDs/VHDXs) residing on the MPIO-managed storage, or presenting storage directly to the VM using in-guest iSCSI initiators or virtual Fibre Channel HBAs (if the storage and Hyper-V host configuration support it), allowing the guest OS to manage MPIO paths independent of the host’s direct hardware passthrough via DDA. The combination of DDA for a LUN and in-guest MPIO for that same LUN via the DDA path is fundamentally incompatible due to how the hardware is presented and managed by each feature.
This behavior is not a bug requiring a fix but is a limitation of the architecture. It highlights that DDA and MPIO, while both related to storage connectivity, serve different purposes and have different requirements for hardware interaction that are mutually exclusive in this specific configuration.
If you have encountered this issue or have insights into related storage configurations in Hyper-V, please share your experiences below.
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