SymFlex: Elastic, Persistent and Symbiotic SSD Caching in Virtualization Environments

Hypervisor managed SSD caching is an often used technique for improving IO performance in virtualization based hosting solutions. Such caches are either explicitly managed by the hypervisor which approximate the access semantics of the applications for improving cache utilization, or operate as statically partitioned devices (which are utilized as caches) by virtual machines. We reason that both these broad directions do not exploit the potential of SSD based IO caches to the fullest, in terms of generalized management policies and performance. We propose SymFlex, a novel method to perform symbiotic management of IO caches by enabling elastic SSD devices. Each virtual machine is configured with an elastic virtual SSD whose contents can be managed according to guest OS and application semantics and requirements. Furthermore, the SSD sizing is managed by the hypervisor with a ballooning-like mechanism to dynamically adjust SSD provisioning to VMs based on performance and usage fairness policies. Our primary contribution of this work is to design and engineer the mechanism for elastic SSD disks to be virtualized, and demonstrate usage models and effectiveness of the symbiotic management of SSD caches across virtual machines. Through our empirical evaluation, we show that the overhead of implementing a virtio-based elastic SSD device is minimal (within 5% of virtio based device virtualization techniques). Further, we demonstrate using dm-cache and Fatcache, the applicability and benefits of SymFlex for enhancing IO throughput and enforcing VM-level SSD allocation policies.