I'm working on preparing my KVM Forum presentation on NBD, and ran into a situation where I really wanted to do the equivalent of 'qemu-img map' over NBD for determining which portions of an overlay image are changed from the backing layer. So after less than 24 hours hacking, I'm pretty pleased with the results.
Known caveats: - Probably has lots of conflicts with Kevin's pending work on refactoring NBD for nicer use in qemu-storage-daemon - Not yet tested with Vladimir's patches to fix bdrv_block_status bugs when the backing file is short (and therefore, applying this series without that is likely to make it possible to expose the same bugs of wrong information) - I _still_ want to get QMP 'block-dirty-bitmap-populate' in qemu 5.2; with that in place, you could avoid the need for this series by instead populating a temporary bitmap and exposing that bitmap over NBD instead. But that requires more work, both in coding (Peter Krempa and I still need to make sure we have the ideal QMP interface) and in usage (managing temporary bitmaps is more effort than a new bool toggle). - And if we _did_ use block-dirty-bitmap-populate, I find myself wanting to be able to expose more than one bitmap at a time over NBD, which in turn means revisiting our current QAPI for nbd-server-add of '*bitmap':'str' to instead allow an alternate type that permits either "'str'" or "['str']", except that our QAPI generator does not yet support arrays in alternates, and is undergoing changes from John Snow for python cleanups... - I am aware of long-standing qemu bugs where when we advertise a large minimum block size (say 4k) but the backing file has smaller granularity (such as 512), then we can violate NBD protocol by sending a reply to NBD_CMD_BLOCK_STATUS with misaligned data. This adds yet another instance of being able to tickle that (rare) bug. I really need to revisit my patches to add bdrv_block_status_aligned... Also available at: https://repo.or.cz/qemu/ericb.git/shortlog/refs/tags/nbd-alloc-depth-v1 Eric Blake (3): nbd: Simplify meta-context parsing nbd: Add new qemu:allocation-depth metacontext nbd: Add 'qemu-nbd -A' to expose allocation depth docs/interop/nbd.txt | 22 ++- docs/tools/qemu-nbd.rst | 6 + qapi/block-core.json | 13 +- include/block/nbd.h | 15 +- blockdev-nbd.c | 3 +- nbd/server.c | 294 ++++++++++++++++++++----------------- qemu-nbd.c | 16 +- tests/qemu-iotests/309 | 73 +++++++++ tests/qemu-iotests/309.out | 22 +++ tests/qemu-iotests/group | 1 + 10 files changed, 310 insertions(+), 155 deletions(-) create mode 100755 tests/qemu-iotests/309 create mode 100644 tests/qemu-iotests/309.out -- 2.28.0
