[Applies against qemu 1.1.0, not HEAD, because qemu from git seems to be broken at the moment. But the patch is easy to rebase.]
This patch adds a -drive detect_zero=on|off flag (defaults to "off"). Setting this on causes the write path to detect all zero byte writes and, for block device drivers which support this, turn the request into the more space-efficient bdrv_co_do_write_zeroes call. The use case is for virt-sparsify. We want to create an intermediate qcow2 overlay file which we know will mostly contain zeroes, and we want qcow2 to handle this without creating a huge empty file. In my testing, the size of the intermediate file is reduced considerably (down to about 1/4 of the original size), making sparsification of large mostly-empty guests possible for the first time. The implementation is very simple-minded. It tests if the entire request is all zero bytes or not. A better implementation would almost certainly be even more space-efficient, but at the cost of splitting some write requests (if that's a problem). Testing: I booted and tested an F16 guest with this patch and the setting "on", without problems. I also modified libguestfs to add this flag, then ran the whole libguestfs test suite with no reported problems. Rich.
