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[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",



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