SVD: A Scalable Virtual Machine Disk Format

Contrary to CPU, memory, and network, disk virtualization is peculiar, for which virtualization through direct access is impossible. We study virtual disk utilization in a large-scale public cloud and observe the presence of long snapshot chains, sometimes composed of up to 1,000 files. We then demonstrate, through experimental measurements, that such long chains lead to virtualized storage performance and memory footprint scalability issues. To address these problems, we present
SVD
, a new virtual disk format. We implemented
SVD
by extending Qcow2, a popular format, and its Qemu driver. We evaluated our prototype, demonstrating that it brings significant performance enhancements and memory footprint reduction. For example,
SVD
improves the throughput of RocksDB by about 48% on a snapshot chain of length 500.
SVD
also reduces the memory footprint by 15×.