Fast Server Deprovisioning through Scatter-Gather Live Migration of Virtual Machines

Traditional metrics for live migration of virtual machines (VM) include total migration time, downtime, network overhead, and application degradation. In this paper, we introduce a new metric, "eviction time", defined as the time to evict the entire state of a VM from the source host. Eviction time determines how quickly the source host can be taken offline, or the freed resources re-purposed for other VMs. In traditional approaches for live VM migration, such as pre-copy and post-copy, eviction time is equal to the total migration time, because the source and destination hosts are coupled for the duration of the migration. Eviction time increases if the destination host is slow to receive the incoming VM, such as due to insufficient memory or network bandwidth, thus tying up the source host. We present a new approach, called "Scatter-Gather" live migration, which reduces the eviction time when the destination host is resource constrained. The key idea is to decouple the source and the destination hosts. The source scatters the VM\´s memory state quickly to multiple intermediaries (hosts or middleboxes) in the cluster. Concurrently, the destination gathers the VM\´s memory from the intermediaries using a variant of post-copy VM migration. We have implemented a prototype of Scatter-Gather in the KVM/QEMU platform. In our evaluations, Scatter-Gather reduces the VM eviction time by up to a factor of 6 while maintaining comparable total migration time against traditional pre-copy and post-copy for a resource constrained destination.