[libvirt] Sparse image volDownload

[Michal Privoznik]

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