Content Look-Aside Buffer for Redundancy-Free Virtual Disk I/O and Caching

Storage consolidation in a virtualized environment introduces numerous duplications in virtual disks and imposes considerable pressure on disk I/O and caching. In this paper, we present a content look-aside buffer (CLB) approach for simultaneously providing redundancy-free virtual disk I/O and caching. CLB attaches persistent fingerprints to virtual disk blocks, which enables detection of I/O redundancy before disk access. At run time, CLB exploits content pages already present in the guest disk caches to service the redundant reads through page sharing, thus eliminating both redundant I/O requests and redundant disk cache copies. For write requests, CLB uses a group invalidating writeback protocol for updating fingerprints to support crash consistency while minimizing disk write overhead. By implementing and evaluating a CLB prototype on KVM hypervisor, we demonstrate that CLB delivers considerably improved I/O performance with realistic workloads. Our CLB prototype improves the throughput of sequential and random read on duplicate data by 4.1x and 26.2x, respectively. For typical read-intensive workloads, such as booting VM and launching application, CLB's I/O deduplication and cache deduplication eliminates 94.9%--98.5% of read requests and saves 50%--100% cache memory in each VM, respectively. Compared with the QEMU's raw virtual disk format, CLB improves the per-disk VM density by 8x--16x. For mixed read-write workloads, the cost of on-line fingerprint updating offsets the read benefit; nevertheless, CLB substantially improves overall performance.