3 ways to realize Virtual-to-Virtual(V2V) migration
V2V migration is to move operating systems and data between virtual machines. Considering the differences of hosts and virtual hardware, VM can migrate between different physical hosts, and the types of these two VMs don’t need to be identical. For example, both KVM to KVM and VMware to KVM can be workable.
There are several ways to realize V2V migration.
V2V offline migration
Offline migration is also known as regular migration or static migration, which requires virtual machine to be suspended before migration. Under shared storage scenario, only the system state will be copied to the destination host, used for later VM rebuilt and job recovery. Under local storage scenario, both image and state of the virtual machine will be copied to the destination host at the same time. From the user's point of view, there is a clear period of time when the service is unavailable. This type of migration method is simple and easy to implement, and is suitable for occasions where service availability is not strictly required.
V2V online migration
Online migration is also known as live migration, which means to migrate VM when service on VM running normally. The virtual machine is still migrating between different physical hosts, and the logical steps are almost the same as offline migration. The difference is that in order to ensure the availability of VM services during migration process, the migration process has only a very short downtime. In the previous stage of the migration, the service runs on the source host. When the migration reaches a certain stage, the destination host will then have the necessary resources to run the system. After a very fast switchover, the source host will transfer the admin role to the destination host, and the service continues to run on the destination host. For the service itself, since the switching time is very short, users will not feel the interruption of the service, so the migration process is transparent to them. Compared to offline migration, online migration is suitable for scenarios that require high service availability.
At present, mainstream online migration tools, such as VMware's VMotion and XEN's xenMotion all require centralized shared external storage devices like SAN (storage area network) and NAS (network-attached storage) between physical machines, so that when migration processing, users only need to consider the migration of operating system memory execution state, so as to obtain better migration performance.
In addition, in some occasions where shared storage is not used, storage block online migration technology can be used to implement V2V virtual machine online migration. Compared with online migration based on shared storage, this type of method requires simultaneous migration of virtual machine disk images and system memory states, which reduces overall migration performance. However, it succeeds to transfer the computer environment under distributed local storage scenario, and ensure the availability of OS services during the migration process, expanding the application scope of online VM migration.
V2V online migration technology eliminates the relevance of software and hardware, and is a powerful tool for software and hardware system upgrades, maintenance and other management operations.
V2V memory migration technology
For the migration of VM memory state, both XEN and KVM adopt the mainstream pre-copy strategy. After the migration starts, the source host VM is still running, while the destination host VM is disabled. The first loop sends all the memory page data from source host VM to the destination host VM, and each subsequent round of the loop later sends the dirty pages of the memory that were written by the VM during the previous round of pre-copy. Until the time is right when the pre-copy cycle ends, the shutdown copy phase will begin to process. The source host will be suspended and produce no more memory updates. Dirty pages in the last round of loop are transferred to the destination host VM. The pre-copy mechanism greatly reduces the amount of memory data that needs to be transmitted during the downtime copy phase, thereby greatly reducing the downtime.
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