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-q... 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 [1] 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 [2]. Martin [1] https://libvirt.org/formatdomain.html [2] https://bugzilla.redhat.com/enter_bug.cgi?product=Virtualization%20Tools&component=libvirt

Thanks! Frank

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