[RFC PATCH 0/2] New NBD metacontext

Eric Blake posted 2 patches 3 days, 4 hours ago
Failed in applying to current master (apply log)
docs/interop/nbd.txt                          | 31 ++++++-
docs/tools/qemu-nbd.rst                       |  4 +-
qapi/block-export.json                        |  4 +-
include/block/nbd.h                           | 10 ++-
nbd/server.c                                  | 87 +++++++++++++++++--
.../{309 => tests/nbd-qemu-allocation}        |  5 +-
.../nbd-qemu-allocation.out}                  | 13 ++-
7 files changed, 139 insertions(+), 15 deletions(-)
rename tests/qemu-iotests/{309 => tests/nbd-qemu-allocation} (95%)
rename tests/qemu-iotests/{309.out => tests/nbd-qemu-allocation.out} (79%)

[RFC PATCH 0/2] New NBD metacontext

Posted by Eric Blake 3 days, 4 hours ago
This is my counter-proposal to Nir's request [1] to revert a 6.0
behavior change.  It does not expose any new information over NBD, but
does make it easier to collect necessary information from a single
context rather than requiring the client to have to request two
contexts in parallel, then cross-correlate what may be different
extent lengths between those contexts.  Furthermore, this is easy to
backport to downstream based on qemu 6.0, at which point clients could
use the existence or absence of qemu:joint-allocation as a witness of
whether it can get away with trusting base:allocation when trying to
recreate a qcow2 backing chain.

[1] https://lists.gnu.org/archive/html/qemu-devel/2021-06/msg01796.html

Things I still want to do:
- a followup patch to libnbd to teach 'nbdinfo
  --map=qemu:joint-allocation' to decode the bits
- teach 'nbdinfo --map' to read all available contexts, instead of
  having to manually type each map besides base:allocation
- potential followup patches to qemu to automatically feed this
  information through qemu-img map:
  - add a new BDRV_BLOCK_BACKING bit for bdrv_block_status(), with
    opposite semantics from BDRV_BLOCK_ALLOCATED, but where the only
    thing known is that the data is not local (not how deep it is)
  - teach qemu to favor qemu:joint-allocation over base:allocation
    when available, and use it to drive BDRV_BLOCK_BACKING
  - teach qemu-img map to recognize BDRV_BLOCK_BACKING

Eric Blake (2):
  iotests: Improve and rename test 309 to nbd-qemu-allocation
  nbd: Add new qemu:joint-allocation metadata context

 docs/interop/nbd.txt                          | 31 ++++++-
 docs/tools/qemu-nbd.rst                       |  4 +-
 qapi/block-export.json                        |  4 +-
 include/block/nbd.h                           | 10 ++-
 nbd/server.c                                  | 87 +++++++++++++++++--
 .../{309 => tests/nbd-qemu-allocation}        |  5 +-
 .../nbd-qemu-allocation.out}                  | 13 ++-
 7 files changed, 139 insertions(+), 15 deletions(-)
 rename tests/qemu-iotests/{309 => tests/nbd-qemu-allocation} (95%)
 rename tests/qemu-iotests/{309.out => tests/nbd-qemu-allocation.out} (79%)

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2.31.1