Re: [libvirt] Changing the default for qcow2 creation

> Only step 4 is timed. I ran the program against test.img prepared in > 4 different ways: > > 1. A sparse raw file: 15.3 seconds > truncate --size 300M test.img > 2. A preallocated raw file: 14.8 seconds > fallocate -l 300M test.img > 3. A sparse qcow2 file: 223.0 seconds > qemu-img create -f qcow2 test.img 300M > 4. A metadata preallocated qcow2 file: 14.5 seconds > qemu-img create -f qcow2 -o preallocated=metadata test.img 300M >

> libvirt's qemu driver doesn't currently preallocate qcow2 metadata > when creating a new image. Given the tiny disk space overhead of the > metadata (0.02%) and the small processing overhead of pre-creation > relative to subsequent creation on-the-fly, I suggest that the > libvirt qemu driver is updated to pre-allocate metadata by default. > > Thoughts? Your test might run faster if you did: $g->copy_size ("/dev/zero", "/test", <size of file>); However I think making the change is a no-brainer. We should add this flag by default.