Install Windows update to Hyper-V failover Cluster. Some VMs will be in "Saved State".

Question

Install Windows update to Hyper-V failover Cluster. Some VMs will be in "Saved State".

Roy Chan 1

In Windows server 2019, all VMs can be migrated to another host.

But in Windows server 2022, after installed windows update to a host, some VMs will be in "Saved State".

Remark: Live migration is ok.

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Answer 1

Henry Mai 2,375 Independent Advisor

Hello Roy, I am Henry and I want to share my insight about this issue

If you're using Windows Server 2022, I'm not sure if you've already tried enabling Processor Compatibility Mode. This feature masks newer CPU instructions, exposing a more generic processor profile to the VM. It's particularly useful for enabling live migration between hosts that may have slight variations in CPU patch levels or microcode. You’ll enable this mode on your critical virtual machines to ensure compatibility and stability during migration.

In Hyper-V Manager, turn off the virtual machine. (This setting can only be changed when the VM is off).
Right-click the VM and select Settings.
Expand the Processor section.
Click on Compatibility.
Check the box that says "Migrate to a physical computer with a different processor version".
Click OK.
Turn the VM back on. It is now configured for greater migration compatibility.

If the problem persists, the cluster log will help us find some hints why the live migration is failing.

Reproduce the issue (e.g., by putting a node into maintenance mode and watching a VM go into a saved state).
Immediately run the following command in an administrative PowerShell on one of the cluster nodes: Get-ClusterLog -Destination -UseLocalTime
This will generate a Cluster.log file in the current directory (C:\Users<username>\Cluster.log).
Open the file and search for the name of the VM that failed to migrate. Look for terms like "Live migration failed", "status," or specific error codes. This log will provide the definitive reason for the failure.

If you're unable to read the log, feel free to upload it. I'll help you review and revise it to identify the underlying issue.

Roy Chan 1 Reputation point

2025-07-16T08:15:42.44+00:00

Hi Henry,

Thanks your for your reply.

The model of CPU processors in two hosts are same, so I dont think it relates to the compatibility issue.

I am wondering if the behaviour is changed in 2022.

I can see all VMs are migrated to another host in Server 2019, but in 2022, some VMs will be in "Saved" state.
Henry Mai 2,375 Reputation points Independent Advisor

2025-07-18T15:13:54.7533333+00:00

Hi Roy, I see your concern. The problem isn't the CPU model, it's the CPU's microcode version at a specific moment in time.

Initial State: Both of your hosts have the same CPU model and the same microcode version. Everything is perfectly compatible.

Windows Update: You begin patching one host. This Windows Update includes a new, updated CPU microcode file from Intel or AMD for security and stability reasons (e.g., to mitigate a vulnerability).

The Mismatch: As soon as the update is applied and the host reboots, that host is now running a slightly different processor version than the other un-patched hosts.

Stricter 2022 Checks: The automated drain process in a Windows Server 2022 cluster is stricter and more security-conscious than in 2019. When it tries to live-migrate a VM, the hypervisor on the destination node detects this microcode mismatch. To guarantee 100% stability and prevent a potential crash, it rejects the live migration.

The "Saved State" Fallback: The cluster's policy during an automated drain is safety first. If a live migration is rejected for any reason, the cluster will place the VM into a Saved State. This preserves the VM's memory and prevents it from being terminated when the host reboots.
Roy Chan 1 Reputation point

2025-07-21T02:06:15.5166667+00:00

Thanks for your reply.

It will happen every month when applying Windows updates, so I dont think it relates CPU microcode as it will not update every month"?
Henry Mai 2,375 Reputation points Independent Advisor

2025-07-21T09:58:11.2566667+00:00

You're right—microcode files don't necessarily require monthly updates. However, the monthly Cumulative Update does patch both the Windows kernel and the hypervisor. When a host reboots with these updates applied, its active kernel may differ from that of the unpatched nodes. During rolling updates, the system identifies these hosts as mismatched, and to maintain stability, it blocks live migration, resulting in a Saved State.

Just recommend you try enabling Processor Compatibility Mode, or review the logs for detailed error messages.
Roy Chan 1 Reputation point

2025-07-23T04:06:49.4366667+00:00

Thanks.

Actually live migration for all VMs are working, so I dont think it relates to compatibility. CPU model of two hosts are identical.

I am just wondering why HyperV will put some VMs to "saved" state rather than live migrate to another host.

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Install Windows update to Hyper-V failover Cluster. Some VMs will be in "Saved State".

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