FYI I added a few enhancements to allow easier tracing inside the file
plugin:
https://github.com/libguestfs/nbdkit/commit/10d4dcfb30cb0ba626a2ec503c7c9...
https://github.com/libguestfs/nbdkit/commit/96a41cd798ec0dce07d97c2a4a04e...
I learned from this that the file_zero function does successfully use
fallocate to zero ranges, at least on my very recent Linux system.
Rich.
--
Richard Jones, Virtualization Group, Red Hat
http://people.redhat.com/~rjones
Read my programming and virtualization blog:
http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines. Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v