KVM Forum 2020

qcow2 is QEMU's native format for disk images. qcow2 images are smaller and more flexible than raw files but are also slower. This problem can be partially mitigated by adjusting the cluster size when creating a new qcow2 image. However there is always a trade-off that needs to be considered: smaller cluster sizes result in smaller images and generally faster allocations but also in more metadata and larger memory requirements. Several approaches have been followed in order to improve this situation. In this presentation we introduce subcluster allocation: a new extension for the qcow2 file format that tries to combine the best of both worlds, producing images that are both faster and smaller.