In the context of

virtualization In computing, virtualization or virtualisation (sometimes abbreviated v12n, a numeronym) is the act of creating a virtual (rather than actual) version of something at the same abstraction level, including virtual computer hardware platforms, stor ...

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

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

(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) 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 The term downtime is used to refer to periods when a system is unavailable. The unavailability is the proportion of a time-span that a system is unavailable or offline. This is usually a result of the system failing to function because of an u ...

). 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 learn ...

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 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 remembered ...

, and its virtual peripherals) are taken in the background by the hypervisor, or by a set of administrative

scripts Script may refer to: Writing systems * Script, a distinctive writing system, based on a repertoire of specific elements or symbols, or that repertoire * Script (styles of handwriting) ** Script typeface, a typeface with characteristics of handw ...

. #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 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