QEMU's default qcow2 L2 cache size is too small for large images (and small cluster sizes), resulting in very bad performance. https://blogs.igalia.com/berto/2015/12/17/improving-disk-io-performance-i... shows huge performance hit for a 20GB qcow2 with default 64kB cluster size: L2 Cache, MiB Average IOPS 1 (default) 5100 1.5 7300 2 12700 2.5 63600 The above link also gives the formula: optimal L2 cache size = L2 table size = (8 Byte) * (disk size) / (cluster size) and the QEMU command line for setting L2 cache size, which is not persistent, but must be specified at each invocation: qemu-system-x86_64 -drive file=hd.qcow2,l2-cache-size=2621440 I'd like to set the l2-cache-size in libvirt xml, but cannot make out how to do it. There are only hints in the comments at the above link and at http://wiki.libvirt.org/page/QEMUSwitchToLibvirt and an earlier unanswered question on this list https://www.redhat.com/archives/libvirt-users/2016-February/msg00036.html Has anyone successfully done this, and could you please give an example of the actual libvirt xml used? Thanks! Frank