  Contrary to the other resources such as CPU, memory, and network, for which
virtualization is efficiently achieved through direct access, disk
virtualization is peculiar. In this paper, we make four contributions. Our
first contribution is the characterization of disk utilization in a public
large-scale cloud infrastructure. It reveals the presence of long snapshot
chains, sometimes composed of up to 1000 files. Our second contribution is to
show that long chains lead to performance and memory footprint scalability
issues by experimental measurements. Our third contribution is the extension of
the Qcow2 format and its driver in Qemu to address the identified scalability
challenges. Our fourth contribution is the thorough evaluation of our
prototype, called sQemu, demonstrating that it brings significant performance
enhancements and memory footprint reduction. For example, it improves the
throughput of RocksDB by about 48% compared to vanilla Qemu on a snapshot chain
of length 500. The memory overhead on that chain is also reduced by 15x.
