[Libguestfs] Support for NBD block status in nbdkit & qemu NBD client

Hi Eric: Here's the problem: We have a VMware VDDK file or snapshot. The virtual size of the snapshot is generally much larger than what's allocated. Copying the snapshot using: nbdkit vddk file=foo.vmdk --run 'qemu-img convert $nbd out' is therefore much slower than it could be. (In the real world version of this we're actually copying the source over the network, but the command line for that is too complicated to show in a simple example). VDDK has an API for querying allocated blocks, see ‘VixDiskLib_QueryAllocatedBlocks’ in https://vdc-download.vmware.com/vmwb-repository/dcr-public/8f96698a-0e7b-... It's awkward to use but Martin (CC'd) has played with it and it does seem to work. The question is if we add Structured Replies + Block Status support to nbdkit & implement it for the VDDK plugin, will ‘qemu-img convert’ above be able to use that to optimize the copy? My brief reading of the qemu sources suggests that something has been implemented here so it likely ought to work? I guess the other question is if there are any limitations around alignment that we should be aware of. It seems as if VDDK uses a larger cluster size for sparseness than its sector size (as does qcow2). Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top