Abstract
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×.