Software-defined consistency group abstractions for virtual machines

In this paper we propose a practical scalable software-level mechanism for taking crash-consistent snapshots of a group of virtual machines. The group is dynamically defined at the software virtualization layer allowing us to move the consistency group abstraction from the hardware array layer into the hypervisor with very low overhead (~ 50 msecs VM freeze time). This low overhead allows us to take crash-consistent snapshots of large software-defined consistency groups at a reasonable frequency, guaranteeing low data loss for disaster recovery. To demonstrate practicality, we use our mechanism to take crash-consistent snapshots of multi-disk virtual machines running two database applications: PostgreSQL, and Apache Cassandra. Deployment experiments confirm that our mechanism scales well with number of VMs, and snapshot times remain invariant of virtual disk size and usage.