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Navigate VMware EVC disadvantages during live migration

Traditional VMware vMotion migration relies on hardware similarities. VMware EVC mode enables live migration despite differences, but it can have an effect on performance.

VMware's Enhanced vMotion Compatibility feature can migrate workloads across dissimilar hardware, but VMware administrators hoping to take advantage of this technology must be aware of the VMware EVC disadvantages, including the risks it can pose to production environments.

A key factor of VMware's success has been that VMware admins can rapidly and seamlessly move workloads around their VMware infrastructures. VMware vMotion, VMware's premier service for the live migration of running VMs from one server to another, works seamlessly in the right conditions. Years after its initial release, the ability to migrate a VM across the state or country to another host, data center or cloud remains impressive.

This VMware staple has been dependable, with few issues for most users. A lot of code is in place to make this happen, but a key part of its success is also the similarity of the hardware on both sides of the migration.

VMotion migration relies on the origin and destination hardware being similar enough to enable the movement of memory maps and CPU instructions in real time. This ensures there aren't any issues with the new location, which is a concern because the migration occurs while the VMs are still running.

VMware introduced Enhanced vMotion Compatibility (EVC) to address the limitation of needing similar hardware. This functionality promised relief, but the potential VMware EVC disadvantages caused concern.

VMware EVC mode enables CPUs of the same family to look and operate similarly enough that the virtual guest is unaware of the CPU differences. This goes beyond CPU speed or number of cores. EVC can mask core CPU functions that are abstracted away by the hypervisor.

When an admin enables VMware EVC mode, the cluster it applies to comes down to the lowest common denominator of the CPUs in the cluster. The hypervisor can't create new CPU functions from something that doesn't already exist on the physical CPU, so the baseline becomes the lowest common point between all of the CPUs in the cluster. Simply put, the hypervisor masks newer features to make a CPU look older.

This is a one-way function that keeps everything even rather than bringing everything up to the highest level, which is impossible. Reducing performance or functionality is one of the primary disadvantages of EVC, but as long as admins prepare for it, it shouldn't be an issue.

Make a plan to overcome VMware EVC disadvantages

VMware EVC mode will reduce a new CPU's functionality, but the gap isn't as wide as a chip generation. Most of the time, EVC requires the CPUs to be in the same CPU family, so the effect might be minimal. This EVC disadvantage isn't often the main reason admins don't use VMware EVC mode, but it's something to be aware of.

Organizations generally don't want to pay for CPU features that aren't in use. Some will argue for buying older CPUs for the sake of compatibility, but the speed of chip advancement makes it difficult and expensive to buy an older CPU because they get phased out after about a year to a year and a half. Unfortunately, stockpiling servers for a VMware cluster is rarely practical due to the initial cost of purchase and the ongoing cost of keeping additional hardware out of production.

The ability to work around some of the rules sounds ideal until the scale and importance of the environment and the consequences of its failure become apparent.

The advantages and disadvantages of VMware EVC are less clear in terms of stability. As fast and complex as a modern hypervisor's capabilities are, the removal or manipulation of CPU features at speeds in the milliseconds or less will create timing issues. This isn't a bug -- errors can occur if high-speed data is manipulated quickly. Of course, VMware and other vendors' hypervisors check for errors to prevent everything from breaking down, but the migration won't be as effective as a migration in which the hardware is truly the same.

The ability to work around some of the rules sounds ideal until the scale and importance of the environment and the consequences of its failure become apparent. That doesn't mean VMware EVC disadvantages can disrupt its role in the modern data center. It's a good tool to use to migrate workloads without having to reboot them. Admins can also use it to extend a cluster's life for the sake of budget concerns.

Despite these use cases, EVC mode should be an exception rather than the rule. Its capabilities are impressive, but unlike a traditional vMotion migration, VMware EVC disadvantages will always create doubt. It's not a native action, and that's a concern for core production systems.

This was last published in August 2018

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