Live migration allows a server administrator to move a running virtual machine or application between different physical machines without disconnecting the client or application. For a successful live migration, the memory, storage, and network connectivity of the virtual machine needs to be migrated to the destination.
Contents |
In memory migration of a VM, usually the Virtual Machine Manager copies all the memory pages from source to destination while the VM is still running on the source. If some memory pages change during memory copy process—dirty pages, they will be re-copied until the rate of re-copied pages is not less than page dirtying rate.[1]
After warm-up phase, the VM will be stopped in source and the remaining dirty pages will be copied to the destination and VM will be resumed in destination.[2] The time between stopping VM on source and resuming it on destination is called "down-time". Down-time of a VM during a live migration could be a few millisecond to seconds according to the size of memory and applications running on the VM.
Post-copy[3] VM migration is initiated by suspending the VM at the source. With the VM suspended, a minimal execution state of the VM (CPU, registers, and non-pageable memory) is transferred to the target. The VM is then resumed at the target, even though the entire memory state of the VM has not yet been transferred, and still resides at the source. At the target, when the VM tries to access pages that have not yet been transferred, it generates page-faults, which are trapped at the target and redirected towards the source over the network. Such faults are referred to as network faults. Source host responds to the network-fault by sending the the faulted page. Since each page fault of the running VM is redirected towards the source, it can degrade the applications running inside the VM. However, when pure demand-paging accompanied with the techniques such as pre-paging can reduce this impact by a great extent.
When down-time of a VM during a live migration is zero or a few millisecond which is not noticeable by end user, it is called a seamless live migration. Otherwise, the end user will feel a small or relatively long glitch in the service.