In the context of

virtualization In computing, virtualization (abbreviated v12n) is a series of technologies that allows dividing of physical computing resources into a series of virtual machines, operating systems, processes or containers. Virtualization began in the 1960s wit ...

, where a ''guest'' simulation of an entire computer is actually merely a software

virtual machine In computing, a virtual machine (VM) is the virtualization or emulator, emulation of a computer system. Virtual machines are based on computer architectures and provide the functionality of a physical computer. Their implementations may involve ...

(VM) running on a ''host'' computer under a

hypervisor A hypervisor, also known as a virtual machine monitor (VMM) or virtualizer, is a type of computer software, firmware or hardware that creates and runs virtual machines. A computer on which a hypervisor runs one or more virtual machines is called ...

, migration (also known as teleportation, also known as live migration) is the process by which a ''running'' virtual machine is moved from one physical host to another, with little or no disruption in service.

Subjective effects

Ideally, the process is completely transparent, resulting in no disruption of service (or

downtime In computing and telecommunications, downtime (also (system) outage or (system) drought colloquially) is a period when a system is unavailable. The unavailability is the proportion of a time-span that a system is unavailable or offline. This is ...

). In practice, there is always some minor pause in availability, though it may be low enough that only

hard real-time Real-time computing (RTC) is the computer science term for hardware and software systems subject to a "real-time constraint", for example from event to system response. Real-time programs must guarantee response within specified time constrai ...

systems are affected. Virtualization is far more frequently used with network services and user applications, and these can generally tolerate the brief delays which may be involved. The perceived impact, if any, is similar to a longer-than-usual

kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learnin ...

delay.

Objective effects

The actual process is heavily dependent on the particular virtualization package in use, but in general, the process is as follows: #Regular snapshots of the VM (its simulated

hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk drive platter, pla ...

storage, its

memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembe ...

, and its virtual peripherals) are taken in the background by the hypervisor, or by a set of administrative scripts. #Each new snapshot adds a differential overlay file to the top of a stack that, as a whole, fully describes the machine. Only the topmost overlay can be written to. #Since the older overlays are read-only, they are safe to copy to another machine—the backup host. This is done at regular intervals, and each overlay need only be copied once. #When a migration operation is requested, the virtual machine is paused, and its current state is saved to disk. #These new, final overlay files are transferred to the backup host. #Since this new current state consists only of changes made since the last backup synchronization, for many applications there is very little to transfer, and this happens very quickly. #The hypervisor on the new host resumes the guest virtual machine. #Ideally, the administrative scripts resume backup operations, the new host becomes the primary, and the previous host now receives the backup copies, readying it for a subsequent migration operation. Note that in practice, regular maintenance operations are required to "merge down" the snapshot stack into a more manageable number of files, to improve performance and optimize storage (redundant overwrites are merged out).

Relation to failover

Migration is similar to the

failover Failover is switching to a redundant or standby computer server, system, hardware component or network upon the failure or abnormal termination of the previously active application, server, system, hardware component, or network in a computer ...

capability some virtualization suites provide. In true failover, the host may have suddenly completely failed, which precludes the latest state of the VM having been copied to the backup host. However, the backup host has everything except for the very latest changes, and may indeed be able to resume operation from its last known coherent state. Because the operations are so similar, systems that provide one capability may provide the other..

References

{{reflist Computer networking Fault-tolerant computer systems