[libvirt] [Qemu-block] [RFC] finegrained disk driver options control

[Alberto Garcia]

On Thu 16 Mar 2017 04:08:50 PM CET, Daniel P. Berrange wrote:
>> OK. How could I change L2 cache size for QCOW2 image?
>> 
>> For 1 Tb disk, fragmented in guest, the performance loss is
>> around 10 times. 10 TIMES. 1000%. The customer could not
>> wait until proper fix in the next QEMU release especially
>> if we are able to provide the kludge specifically for him.
>
> We can explicitly allow L2 cache size set in the XML but that
> is a pretty poor solution to the problem IMHO, as the mgmt
> application has no apriori knowledge of whether a particular
> cache size is going to be right for a particular QCow2 image.
>
> For a sustainable solution, IMHO this really needs to be fixed
> in QEMU so it has either a more appropriate default, or if a
> single default is not possible, have QEMU auto-tune its cache
> size dynamically to suit the characteristics of the qcow2 image.

Related bug report (and discussion):

https://bugzilla.redhat.com/show_bug.cgi?id=1377735

Berto