Comparing the subtle differences between vMotion vs. Live Migration

David Davis

To the novice, VMware's vMotion and Microsoft's Live Migration appear very similar. They both move running virtual machines from one host to another with no (or just a few milliseconds of) downtime. Certainly one vendor would say that its version is superior, but how do they really stack up? Let's take a look at how to approach the vMotion vs. Live Migration debate.

Trying to compare Live Migration and vMotion is similar to comparing fine wines. The subtle differences aren't immediately evident until you spend time with each. Not many people have the time or resources to spend time with both vMotion and Live Migration, so I'll help explain these subtle differences so you can see them much quicker.

A short history of VM migration

In 2003, VMware launched GSX and vCenter, which included vMotion. VMware's vMotion was really the first "ah ha" feature of server virtualization. The vMotion process just seemed so easy that it was unbelievable. I remember testing the process -- using the VM while it was moving and checking the location of the VM before and after -- before I really believed that it had moved. It wasn't just me; VMware's vMotion sold many IT pros on server virtualization.

Microsoft offered what it called Quick Migration with Windows Server 2008 Hyper-V. With Quick Migration, the VM was paused, memory was written to disk, the VM's memory was moved to another Hyper-V host, and it was resumed. This took time to complete and the VM was down during the process.

With Windows 2008 R2, Microsoft replaced Quick Migration with Live Migration. With Live Migration, the VM actually stays available while the VM's memory is moved from one Hyper-V host to another. As with previous versions of vSphere, Live Migration required shared storage.

What's new with vMotion vs. Live Migration?

Shared storage is no longer required for Live Migration in Windows Server 2012. Additionally, with Hyper-V 2012, you can perform more than one Live Migration at a time. With this new feature, an admin can store VMs locally or even on Server Message Block storage and still be able to move VMs without downtime. Live Migration with Windows Hype-V 2012 also supports 10 Gigabit Ethernet (GbE) networks.

The last, and most controversial, aspect to Windows 2012 Hyper-V Live Migration is that Microsoft claims an organization can perform an unlimited number of Live Migrations at once.

VMware made improvements in vSphere 5.1 that combines vMotion and storage vMotion into a single operation. The end result is that vMotion can be performed without shared storage, giving vSphere a capability similar to Hyper-V 2012's Live Migration. This new enhanced vMotion is only available from the vSphere Web client and is included with vSphere Essentials Plus and above. VMware says the maximum number of concurrent vMotions per host is four, with a 1 GbE network connection, and eight with a 10 GbE network connection

Performance differences

In the end, the major differences between vMotion and live migration come down to performance and packaging.

VMware's vMotion is available with vSphere Essentials Plus, and up. Microsoft's Live Migration, on the other hand, is available in the free version of Hyper-V Server 2012. The free version doesn't have a local graphical user interface, but does include the same advanced features found in the commercial version.

Microsoft's claim of unlimited concurrent live migrations has set off a debate about whether such a claim is realistic or ridiculous. Stating that your software has unlimited scale is highly suspect, as many experts have pointed out.

This has led VMware experts, including Chris Wahl and Eric Gray, to poke back, pointing out that unlimited concurrent Live Migrations wouldn't be wise or practical. Both experts say that smart software companies shouldn't say their product can "scale indefinitely" and instead should provide supportable, tested configuration maximums.

In a research study that VMware commissioned, Principled Technologies Inc. found that the average vMotion in vSphere 5 completed 5.4 times faster than the average Live Migration in Windows 20008 R2 SP1. The study also found that Live Migration affected the performance of applications and operating systems more than vMotion. A similar study has not compared Windows 2012 Hyper-V Live Migration and vSphere 5.1 vMotion, nor has Microsoft stated that these performance impacts have been remedied.

If these performance differences still exist between Live Migration and vMotion, and were applied across a mass live migration at a large enterprise or service provider, these performance differences would be compounded and could cause server performance degradation.

vMotion vs. Live Migration: Conclusion

As with many comparisons, the difference may come down to which vendor you trust the most. VMware vSphere has certainly been around the longest, has a larger market share, and has won the faith and trust of countless data center and server managers. Microsoft's Live Migration, on the other hand, is the "challenger" in this picture and still has to win the faith and trust of IT pros around the world. However, just because Live Migration is newer doesn't mean that it's any less trustworthy. 

As with many products or features that appear similar, you don't really know how scalable or trustworthy they are until you stress them. For example, two racecars may look similar, but you really don't know how fast they are or how long they can sustain that speed without putting them on the track. With that in mind, if you are interested in Hyper-V 2012, then I encourage you to test it for yourself and see what you think. However, keep in mind that moving running VMs is just one of the many aspects of a total hypervisor package. After all, the virtualization debate is much more than just vMotion vs. Live Migration. You should look at it as a comparison of vSphere and vCloud Suite vs. Windows Server 2012 and System Center 2012.

View the next item in this Essential Guide: Hyper-V Live Migration impresses VMware loyalist or view the full guide: Live Migration vs. vMotion: A guide to VM migration

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Which vendor has the edge when it comes to VM migrations?

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