This content is part of the Essential Guide: Live Migration vs. vMotion: A guide to VM migration

Hyper-V Live Migration terms: brownout, blackout and dirty pages

You may not know about brownouts, blackouts and dirty pages in Hyper-V Live Migrations, but they are useful for monitoring virtual machine live migration.

For several years, VMware has included live migration technology with its vMotion feature. Only recently, Microsoft caught up with its Hyper-V Live Migration feature.

Hyper-V Live Migration is undoubtedly the most sought-after feature of Hyper-V because of its ability to move virtual machines (VMs) between clustered hosts without noticeable service interruption. But in fact, Live Migration can cause brief disruptions in service that end users may not notice. As an admin, you should understand some lesser-known Hyper-V Live Migration terms that help monitor and troubleshoot service interruption.

Hyper-V event logs contain information about live migration disruptions that can briefly affect VMs. For every VM live migration, these logs report three events: a brownout event, a blackout and dirty-pages event, and a summary of the live-migration process. There is not enough information available on these terms, but they are important when outlining the stages of a live migration. Understanding these terms also helps you troubleshoot live migrations that take too long and prevent administrative tasks.

In this article, I explain how to use Hyper-V logs and outline where to locate logs, what the terms mean, and why you need this information for a successful live migration.

Where to find Hyper-V Live Migration event logs
To produce reports, you need to perform a live migration using Hyper-V R2 with Failover Cluster Administrator, System Center Virtual Machine Manager or scripting.
You'll find the Live Migration logs in the Application and Services Logs section of the Windows Server 2008 event viewer. Follow this path: Application and Services Log -> Microsoft -> Windows -> Hyper-V-Worker.

Figure 1
The Hyper-V-Worker event log (Click image for an enlarged view.)

Once you have located the Hyper-V-Worker event log (see Figure 1), right-click on the admin log and filter by event number. These Hyper-V Live Migration terms are numbered as follows: brownout event (22508), blackout and dirty-pages event (22509), and the live migration summary event (22507).

A Live Migration brownout event 
A Hyper-V-Worker event log lists the brownout stage first. In the context of virtualization, a brownout is defined as the amount of time it takes to complete the memory-transfer portion of Hyper-V Live Migration. And the term brownout is a good metaphor for this event, because a VM is not affected completely (as the term blackout suggests). The VM is still responsive, but you can't perform configuration changes or other administrative functions during this stage of the live migration.

Figure 2
The brownout event (Click image for an enlarged view.)

Figure 2 indicates that the brownout took 19.43 seconds. This time depends on the size of active RAM the VM uses and the speed of the Live Migration transport network. During this time, the VM is completely responsive as the memory pages move to the destination node. This stage of live migration gets most of a VM's state over to another node, but not quite all. Since the VM is responsive, users most likely never know that a migration to another node is in process. But VM response may get delayed. You can monitor this delay by constantly pinging the VM with the command ping SERVERNAME --t. You'll notice brief periods of longer response times, without total disruption of service.

Live Migration blackout and dirty-pages event
The final stage in Hyper-V Live Migration is when a VM fully migrates to the destination node of the cluster. This process is called the blackout stage, where, to finally move a VM and all its memory, there is a brief pause in service. During the brownout stage, the host attempts to move all active memory to the destination node. But server memory isn't completely emptied until this final process, where data is moved to the destination node. A final snapshot provides a last file representation of the remaining memory, which is known as dirty pages. When dirty pages are migrated to the destination node, the blackout occurs.

The blackout period is by no means comparable to the longer saved state in Hyper-V's former Quick Migration feature, because Live Migration usually moves a very small amount of data during this final stage. But a slight disruption will occur, usually about one to two seconds, or one dropped ping. Unlike during the brownout stage, a VM is not responsive.The event log indicates how long the blackout period was and how many dirty pages were moved during the migration's final stage (see Figure 3).

Figure 3
The blackout and dirty-pages event (Click image for an enlarged view.)

Note that during a live migration for servers with a higher transaction workload, longer blackout times and a greater number of dirty pages occur.

These two Hyper-V Live Migration terms are important, because the blackout and dirty-pages event are troubleshooting tools. The log tells you for how long a VM was unavailable, which is useful information when a live migration takes longer than expected or when there is a noticeable disruption in service.

Live Migration summary event
The final event, 22507, gives a nice summary of the duration of the live migration process.

Figure 4
Live Migration summary event (Click image for an enlarged view.)

To determine exactly which server experienced the brownout and blackout, run a simple System Center Virtual Machine Manager PowerShell script. In the following command, insert the VM globally unique identifier found in the summary event:

get-vm | where{$_.VMid -eq "Enter VM GUID here"} | ft name

With any technology, understanding the key details of how a process works is important to the long-term stability of your environment. These events may just be informational, but the inner workings of Hyper-V Live Migration come in handy when troubleshooting disruptions. If you're upgrading or changing the Live Migration network interface card, these logs also help you compare before-and-after results to determine whether the change improved your environment.

About the expert
Rob McShinsky is a senior systems engineer at Dartmouth Hitchcock Medical Center in Lebanon, N.H., and has more than 12 years of experience in the industry -- including a focus on server virtualization since 2004. He has been closely involved with Microsoft as an early adopter of Hyper-V and System Center Virtual Machine Manager 2008, as well as a customer reference. In addition, he blogs at, writing tips and documenting experiences with various virtualization products.


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