These faults, known as network faults, are trapped at the target and redirected to the source, which responds with the faulted page. At the target, if the VM tries to access a page that has not yet been transferred, it generates a page-fault. Concurrently, the source actively pushes the remaining memory pages of the VM to the target - an activity known as pre-paging.
With the VM suspended, a minimal subset of the execution state of the VM (CPU state, registers and, optionally, non-pageable memory) is transferred to the target. Post-copy VM migration is initiated by suspending the VM at the source. There are some techniques to reduce live migration down-time, such as using probability density function of memory change. The time between stopping the VM on the original host and resuming it on destination is called "down-time", and ranges from a few milliseconds to seconds according to the size of memory and applications running on the VM. Stop-and-copy phase Īfter the warm-up phase, the VM will be stopped on the original host, the remaining dirty pages will be copied to the destination, and the VM will be resumed on the destination host. If some memory pages change (become 'dirty') during this process, they will be re-copied until the rate of re-copied pages is greater than the page dirtying rate. In pre-copy memory migration, the Hypervisor typically copies all the memory pages from source to destination while the VM is still running on the source. Two techniques for moving the virtual machine's memory state from the source to the destination are pre-copy memory migration and post-copy memory migration.
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