When virtualization first came onto the scene, one of the “wow factors” was the ability to be able to seamlessly move virtual machines between physical hardware. This is still one of the powerful features today of running production workloads on top of hypervisors – the ability to relocate business-critical resources quickly and efficiently. With Windows Server Hyper-V, there are many mechanisms built into the platform that allow it to serve out highly available production workloads and allows resources to be easily migrated between physical hardware.
In this post, we will take a look at Hyper-V Cluster Virtual Machine Migrations and the ability to shift around resources to serve the purposes of high availability and distribution of resources.
Hyper-V Cluster Virtual Machine Migration
There are a few migration options for Hyper-V administrators when it comes to migrating resources between physical hardware in a Hyper-V Cluster.
We will take a look at the following options in greater detail and see how each play into the intended use case for the functionality.
- Quick Migration
- Live Migration
- Storage Live Migration
- Shared Nothing Live Migration
As shown, Hyper-V offers a good number of migration technologies that Hyper-V administrators can take advantage of when ensuring production virtual machine workloads are highly available and are reachable to end users. These various technologies provide a number of tools to utilize. However, there are certain requirements and caveats to a few of the migration technologies to be aware of.
Hyper-V Quick Migration
One of the older forms of migration technology with Microsoft Hyper-V is the feature called Quick Migration. This allows “quick” migrations between Hyper-V hosts. The caveat to this type of Hyper-V migration is that it incurs a short interruption to the service of a virtual machine. With Hyper-V Quick Migration, Hyper-V places the virtual machine in what is called a “saved state” in shared storage between Hyper-V cluster hosts. This is similar to placing the virtual machine in a state of hibernation.
Hyper-V saves the state of the virtual machine, including memory to shared storage. It is during this phase of the process the virtual machine is inaccessible to end users or is unavailable on the network.
Hyper-V then unregisters the virtual machine from one Hyper-V host and then registers it on another Hyper-V host. The virtual machine is then resumed into operation after the registration process on the new host finishes. The Speed of this operation is generally relative to the amount of memory the virtual machine has configured as well as the speed of the storage connected to the Hyper-V hosts and bandwidth of the connections.
Hyper-V Live Migration
The next type of migration technology to introduce is the well-known Live Migration functionality which was introduced in Windows Server 2008 R2. Live Migration is very similar to the VMware vMotion technology that allows for very little to no interruption of network connectivity in a virtual machine to allow transferring the active memory, compute, and network components from one Hyper-V host to another.
The interruption to virtual machine connectivity is much less impact to the end user than with the Quick Migration process. With Live Migration, the virtual machine is never placed in a saved state condition where memory is copied to disk. The only interruption incurred is when MAC addresses and ARP tables have to momentarily be updated to reflect the change in the uplink for the virtual machine that is moved between Hyper-V hosts.
Requirements for Hyper-V Live Migration in Windows Server version prior to Windows Server 2016, require Failover Clustering. With Windows Server 2016, it no longer requires Failover Clustering to work. Additionally, with Live Migration, you will want to make sure the processor and chipsets are very similar if not identical between the source and destination Hyper-V hosts. Configuring a Live Migration network preferred for Live Migration traffic is preferable, although Hyper-V will try a number of different networks including the cluster network to attempt the transfer.
Hyper-V Live Migration provides the cornerstone migration technology for many of the Hyper-V high availability mechanisms, including draining roles from a Hyper-V host. In contrast to the Quick Migration functionality, Live Migration depends on the cluster network being available in a Hyper-V cluster. Quick Migration does not rely on any cluster network.
During the live migration process, memory pages are transferred from the source Hyper-V host to the destination Hyper-V host. This is copied over the designated Live Migration network assigned in the Windows Failover Cluster configuration. This working set of memory is transferred to the destination host and any modified pages are tracked during the process. The modified memory pages are copied across to the destination Hyper-V host. Once the modified pages are transferred to the destination host, the storage is then handed over to the destination Hyper-V host. The destination Hyper-V host now has the complete working virtual machine and is ready to assume ownership and operation of the VM. Hyper-V sends a message to the physical network that causes the MAC address for the virtual machine to be re-learned so network traffic is continued for the VM as well.
Hyper-V Storage Live Migration
Hyper-V also provides the ability to migrate a virtual machine between storage while the virtual machine is running. This functionality was introduced in Windows Server 2012 and is called Storage Live Migration. Those who are familiar with VMware’s Storage vMotion technology will be familiar with the process that Storage Live Migration offers. The virtual hard disks including VHD and VHDX are moved to different storage devices in a way that allows the virtual machine to stay up and running and accessible to end users.
During the Storage Live Migration process, the virtual hard disk is mirrored to another attached storage device. This can include a different folder on the same disk, different LUN, from DAS to SAN or from SAN to an SMB file share to name a few. The trick with Storage Live Migration is that the virtual machine isn’t really moving between hosts, only the storage is moving.
The process to perform the Storage Live Migration includes the following. The source VHD or VHDX files are copied from the source to the target location. When the copy is started, all the new writes are performed on the source and target VHD through a mirroring process. When the copy or mirror operation is complete, the virtual machine is switched over to use the VHD or VHDX files on the target location. Once the mirror operation is finished and deemed successful by Hyper-V and the host can successfully connect to the virtual machine files in the new storage, the VHD or VHDX files and configuration files are deleted from the source.
Hyper-V Shared Nothing Live Migration
With other Hyper-V migration technologies that we have covered, they require things to be in common between Hyper-V hosts, such as shared storage. However, Windows Server 2012 also introduced the Shared Nothing Live Migration. This allows a true migration of a running virtual machine from one Hyper-V host to another Hyper-V host with network connectivity being the only requirement.
This allows moving virtual machines that are not part of a cluster to another Hyper-V host that is not clustered or to a non-clustered host to a host that is part of a cluster. Once the virtual machine is housed in the cluster environment, it can then be made highly available through the normal means. Keep in mind that you will have to make sure the virtual machine that is not highly available before performing the Shared Nothing Live Migration.
Concluding Thoughts
Hyper-V provides some really powerful migration technologies for use in making sure Hyper-V virtual machines are highly available as well as mobile for performing planned and unplanned maintenance on Hyper-V physical hosts. Over the various Hyper-V versions, Microsoft has introduced new and more powerful migration technologies. These include Quick Migration, Live Migration, Storage Live Migration, and Shared Nothing Migration. Regardless of the scenario or connected hardware, it is possible to accommodate just about any migration scenario even if there is no shared storage or if you need to move virtual machines between storage devices. With the exception of the Quick Migration process, all of the covered migration technologies allow moving a running virtual machine without any downtime.
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