[libvirt] Sparse image volDownload

Dear list, I'd like to hear your opinion on the following bug: https://bugzilla.redhat.com/show_bug.cgi?id=1282859 Long story short, Imagine the following scenario: 1. Create 4GB file full of zeroes 2. virsh vol-download it What happens is that all those 4GB are transferred byte after byte through our RPC system. Not only this puts needles pressure on our event loop, it's suboptimal for network and other resources too. I'd like to explore our options here keeping in mind that the original volume might have been sparse and we ought to keep it sparse on the destination too. In the bug the reporter (Matthew Booth) suggests introducing new type of RPC message that will let us keep our APIs unchanged. The source will scan the file for windows of zeroes bigger than some value. When found the new type of message is passed to client without need to copy those zeroes. Yes, this is very similar to RLE. If we are going that way, should we enable users to put a compression program in between read()/write() and our RPC? Well, should we let users to choose what compression program we will put there? Because there are better compression algorithms than RLE. Michal