Let's get this straight: VM live migration

As live migration features have evolved over the years, the terminology around the technology has become more confusing. Let us help you sort it out.

Live migration technology made a splash with its debut 10 years ago. VMware publicly released GSX Server 2.5, introducing IT pros to vMotion, its new feature that sold many IT pros on VMware virtualization. The ability to move running virtual machines from one host to another without downtime was, in 2003, seen as a landmark development for virtualization. A decade later, the ability to seamlessly migrate virtual machines (VMs) is now an integral part of nearly every virtualization deployment.

LGTS logoOver the years, competitors such as Microsoft have added similar capabilities, and VMware has refined vMotion, providing the ability to migrate running VMs from one host to another, even if those hosts do not share the storage device where the VM is located. But these feature additions and changes have also led to some confusion over terminology-- especially since Microsoft calls its competing feature Live Migration, a term many use to refer to the generic process of moving VMs among hosts without downtime. With so many similar terms floating around, it can be easy to get lost when it comes to VM live migration terminology, so let's get this straight.

General live migration terminology

Live migration. As a generic term, live migration (not to be confused with Microsoft's Live Migration) refers to the process of moving VMs among physical hosts without downtime. A VM live migration allows administrators to perform maintenance or resolve a problem on a host without affecting the end-user experience. To understand how live migration works, it's first important to remember that a VM is really made up of two basic components: The VM's storage (the virtual hard disk) and the VM's configuration or state. Often, a VM's storage is located on a storage area network (SAN), and its configuration or state is what's running in a host server's processor and memory. With the traditional process of a live migration, the VM's state and configuration is copied from one physical host to another, but the VM's storage doesn't move.

Live storage migration. As it sounds, storage migration is the process of moving a VM's storage -- that is, its underlying file system -- from one storage location to another. Live storage migration indicates that this transfer happens without affecting the VM.

Shared-nothing live migration. Both Microsoft and VMware offer features that allow admins to move VMs among physical hosts, even if the VM's file system resides on a storage device that is not shared by both hosts -- on a server's local direct-attached storage, for example. At a simple level, a shared-nothing live migration is a combination of traditional VM live migration and storage migration. The VM's state and configuration is copied to a destination host, and then the file system is moved to the destination storage device. To prevent downtime, the VM's state and storage remain running on the original host and storage location until the copying process is completed.

It's important to note that the term live migration can sometimes be used as a catchall phrase to also describe shared-nothing live migration, because this process is often an additional option of a hypervisor's live migration capability rather than a separate feature.

VMware live migration jargon

Storage vMotion. In ESX Server 3.5, VMware released Storage vMotion, a live storage migration feature that allows the migration of a VM's file system from one storage location to another without downtime. In a VMware Storage vMotion, the VM continues to run on the host server, but the files that make up the VM are moved among storage arrays or LUNs. Storage vMotion is especially useful for storage platform upgrades -- when an organization gets a new SAN, for example.

VMware vMotion. VMotion has seen several changes over the years, but it has always allowed the movement of VMs among physical hosts. Previous changes include support for simultaneous vMotions and the ability for VMs to move between hosts with different CPUs (a feature called Enhanced vMotion Compatibility). In vSphere 5.1, VMware added the capability to seamlessly combine vMotion and Storage vMotion into a process that allows for shared-nothing live migration.

Microsoft-specific live migration features

Hyper-V Quick Migration. Microsoft introduced Quick Migration in Windows Server 2008 Hyper-V. While Quick Migration simplified the process of moving VMs among hosts, it wasn't quite live migration, meaning there was some downtime associated with the transfer. With Quick Migration, VMs on the original host pause shortly before the destination host takes over.

Hyper-V Live Storage Migration. This Hyper-V feature was called Quick Storage Migration before Windows Server 2012. As you would expect, Live Storage Migration allows admins to move all or part of a VM's file system from one storage location to another without downtime.

Hyper-V Live Migration. Microsoft replaced Quick Migration with Live Migration in Windows Server 2008 Hyper-V R2. True to its name, Live Migration eliminated the need for downtime when moving VMs among Hyper-V hosts. In Windows Server 2012 Hyper-V, Microsoft added the ability to perform a shared-nothing live migration.

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