Implementing Virtual Machine Replication: A Case Study using Xen and KVM

Virtual machine (VM) replication has been recognized as an inexpensive way of providing high availability on commodity hardware. Unfortunately, its impact on system performance is far from negligible and strategies have been proposed to mitigate this problem. In this paper we take a look at VM replication from a different perspective: the choice of a hypervisor. Namely, the differences between hypervisors in terms of architecture and performance are well known and studied in the literature, but no analysis has been performed so far in the context of replication. Taking open-source hypervisors Xen and KVM as examples, we show what hypervisor services are necessary to implement a primary-backup replication scheme and how hypervisor design affects the development steps and the performance. Interestingly, our userspace implementation on top of KVM achieves roughly the same performance as an already existing, more mature Xen implementation, which leads us to the conclusion that the inherent cost of the replication scheme dominates the differences between the chosen hypervisors.