[Libguestfs] Appliance image. Why raw?

I noticed that raw is intentionally used as image format for appliance image. So I would like to ask, is there any reason to use raw as image format for appliance? I would suggest that choice is related with the performance, so I did several test runs (./run utils/boot-benchmark/boot-benchmark) /appliance/root.raw: Result: 1342.9ms ±2.9ms Result: 1370.1ms ±17.3ms Result: 1365.1ms ±11.0ms appliance/root.qcow2: Result: 1353.1ms ±2.4ms Result: 1350.2ms ±5.3ms Result: 1374.7ms ±5.4ms But even theoretically there should not be much difference, because qcow2 uses a cache to internal tables. Also -drive configured to use the host cache: if (has_appliance_drive) { ADD_CMDLINE ("-drive"); ADD_CMDLINE_PRINTF ("file=%s,snapshot=on,id=appliance," "cache=unsafe,if=none,format=raw",