[libvirt] [RFC] add option to configure qcow2 cache sizes in domain xml

Hi, all. I want to add option to configure qcow2 cache size in domain xml. Looks like this should be a driver option. As driver already has a lot of attributes and even qcow2 has more then one cache I suggest something like this: <driver type="qemu" type="qcow2"> <cache name="l2" size="1024" unit="KiB"/> <cache name="refcount" size="256" unit="KiB"/> </driver> This looks compact but qcow2 has entry size option for cache which don't fit nicely in this approach (you have to use same units for both values). So may be this is better: <driver type="qemu" type="qcow2"> <cache name="l2"> <size unit="KiB">1024</size> <entrysize unit="KiB">4</entrysize> </cache> <cache name="refcount"> <size unit="KiB">256</size> </cache> </driver> Or may be start specifying size and units in one attribute: <driver type="qemu" type="qcow2"> <cache name="l2" size="1MiB" entrysize="4KiB"/> <cache name="refcount" size="256KiB"/> </driver> Also qemu has a feature to specify only one value for caches so that qemu sets l2 and refcount caches in hardcoded 4:1 ratio which is good for images with default refcount_bits=16. On one hand this is nice to have in libvirt xml too so that user can specify only one memory size. On another hand "there's no strict need for both caches to cover the same amount of disk space" [1] and assuming user tuning cache sizes know what he is doing we can live without this option in libvirt xml. I tend to not to add this option to libvirt xml. [1] qemu/docs/qcow2-cache.txt Nikolay