New Cluster-Wide Control For Virtual Machine Live Migrations In Windows Server and Azure Stack HCI

January 6, 2023

Applies to: Windows Server 2022, Azure Stack HCI, version 21H2 and later versions of both

Overview:

There is a new enhancement in the ability to manage the number of parallel live migrations within a cluster, making it easier to change and ensuring consistency. Previously, changing it required setting it on each node of the cluster, and remembering to set it when a new server is added to the cluster. This meant it was easy to have inconsistencies across the nodes.

Hyper-v has a setting to limit the number of live migrations that a server can participate in. If an administrator wanted to change this value to be optimized for their systems, they would have to go to each node of a failover cluster and change the per-server Hyper-V property. They would also have to remember to set this property for any new node added to the cluster. This meant that it was difficult to ensure consistency over time.

With the installation of the September 2022 Windows Update package or later, The new cluster property MaximumParallelMigrations was added that allows an administrator to set the value once and have each node of the cluster inherit the setting. When new servers are added to the cluster, the cluster value will be inherited. This ensures consistency and makes it easy to adjust the system.

Using the new property:

The default when the update is applied is MaximumParallelMigrations=1.

To get the current value the cluster property:

(Get-Cluster).MaximumParallelMigrations

The value can be changed by using the command:

(Get-Cluster).MaximumParallelMigrations=<value>

To view the nodes property:

Get-VMHost -ComputerName <name1>,<name2>,<name3> | FT Name, MaximumVirtualMachineMigrations

More Information

The first time a monthly update with this change is applied, the local MaximumParallelMigrations setting will be converted to a cluster level setting and set to 1. Based on testing this is the recommended default with the safest and most reliable value as far as reliability with live migrations across the various types of systems deployed today. If this parameter is changed by an administrator after it is added to the system via a monthly update, the new value will persist.

We recommend you test and validate the reliability of more than one parallel live migration with your hardware and in your environment before setting to a higher number.

References

Optimizing Hyper-V Live Migrations on an Hyperconverged Infrastructure - Microsoft Tech Community

Set-VMHost (Hyper-V) | Microsoft Learn

2022-09 Cumulative Update for Azure Stack HCI, version 20H2 and Windows Server 2019 Datacenter: Azure Edition for x64-based Systems (KB5017311)