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 [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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