Virtual machine live migration is a virtualization process that moves a virtual machine (VM) from one physical host server to another. It moves the memory and state of a VM without shutting down the application, so users will generally not detect any significant interruption in application availability.
The process captures the complete memory space occupied by the VM—along with the exact state of all the processor registers currently operating on the VM—then sends that content across a TCP/IP link to memory space on another server. Processor registers are then loaded, and the newly moved VM can pick up its operation without missing a step.
Most VM live migrations occur between similar hypervisors, so the migrated VM retains its name and other unique identifiers. Even though the VM is on a different server, it's the exact same machine as far as the users are concerned.
Live migration is a key benefit of virtualization, allowing workloads to move in real time as server or data center conditions change. Consider the impact on business continuity: A virtual server scheduled for maintenance can migrate its workloads to a spare server or to other servers that have extra computing capacity. Once the maintenance is complete and the server returns to service, these workloads can all migrate back to the original server without disruption.
Live migration helps server consolidation by allowing IT administrators to balance workloads across data center servers, ensuring that each server is used efficiently without being overtaxed. Live migration helps with disaster recovery too because VMs can just as easily be moved from one site to another, relying on spare servers at a remote site to receive and operate the migrated VMs.
All of the major virtualization software platforms include VM live migration tools. These include VMware VMotion (part of vSphere), Microsoft Live Migration (part of Hyper-V R2) and Citrix Systems XenServer live migration.
Migration tools typically allow administrators to prioritize the movement of each VM so that failover and failback processes occur in a predictable and repeatable manner. Mission-critical VMs usually take priority and are often moved to spare servers with ample computing resources.
Secondary VMs can be addressed next, although the migration software may be left to move noncritical VMs automatically based on the computing resources on each available server. Migration audits allow administrators to locate VMs and track their movements to refine and optimize ongoing migration behaviors.
Live migration works between almost all virtual host servers, but it's important to test migration behaviors between servers with various processor manufacturers. Processors from Intel and AMD both include extensions that provide hardware assistance for virtualization tasks, including migration. However, Intel VT and AMD-V processors use different architectures to facilitate migration, and moving VMs between Intel and AMD-based servers may result in unexpectedly poor migration performance.
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