On Mon, Sep 19, 2016 at 11:32:26AM -0400, Frank Myhr wrote:
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-in-qemu-2-5-with-the-qcow2-l2-cache/ 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?
If you don't see it in the docs  it's not supported. It would be trivial to add that option, though. Patches are welcome ;) If you don't feel like it, you can always file an upstream bugzilla request . Martin  https://libvirt.org/formatdomain.html  https://bugzilla.redhat.com/enter_bug.cgi?product=Virtualization%20Tools&component=libvirt
Thanks! Frank _______________________________________________ libvirt-users mailing list email@example.com https://www.redhat.com/mailman/listinfo/libvirt-users
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