[Qemu-devel] [PATCH v6 00/14] block: incremental backup transactions using BlockJobTxn

[Fam Zheng]

v6: Address comments from Max and Eric (many thanks for reviewing!):
    Add Max's reviews.
    Don't leak txn. [Max]
    Unusual comment ending "**/" -> "*/". [Eric]
    Fix the stale block_job_txn_prepare_to_complete comment. [Max]
    Move "block_job_txn_unref" to below FOREACH block. [Max]
    Split "base" and "txn" in qapi schema, so that transactional-cancel is only
    visible in transaction. [Eric]

v5: Address comments from Max Reitz:
    2.4 -> 2.5 in qapi docs.
    Don't leak txn object.
    English syntax fixes.
    Really leave the "cancelled" status of failed job.
    Remove a superfluous added line.

v4: Address comments from John and Stefan, including:
    Rename function: backup_cleanup_sync_bitmap.
    Rename and add comments for .commit and .abort.
    Call .commit and .abort for both txn and non-txn.
    Drop patch 9 in v3.
    Improve the txn implementation. Acquire context locks as accessing them.
    Add refcnt patch for BlockJob.
    Adjust txn unit test code.

This is based on top of the work by Stefan Hajnoczi and John Snow.

Recap: motivation for block job transactions
--------------------------------------------
If an incremental backup block job fails then we reclaim the bitmap so
the job can be retried.  The problem comes when multiple jobs are started as
part of a qmp 'transaction' command.  We need to group these jobs in a
transaction so that either all jobs complete successfully or all bitmaps are
reclaimed.

Without transactions, there is a case where some jobs complete successfully and
throw away their bitmaps, making it impossible to retry the backup by rerunning
the command if one of the jobs fails.

How does this implementation work?
----------------------------------
These patches add a BlockJobTxn object with the following API:

  txn = block_job_txn_new();
  block_job_txn_add_job(txn, job1);
  block_job_txn_add_job(txn, job2);

The jobs either both complete successfully or they both fail/cancel.  If the
user cancels job1 then job2 will also be cancelled and vice versa.

Jobs objects stay alive waiting for other jobs to complete, even if the
coroutines have returned.  They can be cancelled by the user during this time.
Job blockers are still in effect and no other block job can run on this device
in the meantime (since QEMU currently only allows 1 job per device).  This is
the main drawback to this approach but reasonable since you probably don't want
to run other jobs/operations until you're sure the backup was successful (you
won't be able to retry a failed backup if there's a new job running).


Fam Zheng (6):
  backup: Extract dirty bitmap handling as a separate function
  blockjob: Introduce reference count
  blockjob: Add .commit and .abort block job actions
  blockjob: Add "completed" and "ret" in BlockJob
  blockjob: Simplify block_job_finish_sync
  block: Add block job transactions

John Snow (4):
  qapi: Add transaction support to block-dirty-bitmap operations
  iotests: add transactional incremental backup test
  block: rename BlkTransactionState and BdrvActionOps
  iotests: 124 - transactional failure test

Kashyap Chamarthy (1):
  qmp-commands.hx: Update the supported 'transaction' operations

Stefan Hajnoczi (3):
  blockdev: make BlockJobTxn available to qmp 'transaction'
  block/backup: support block job transactions
  tests: add BlockJobTxn unit test

 block.c                    |  19 ++-
 block/backup.c             |  50 +++++--
 block/mirror.c             |   2 +-
 blockdev.c                 | 354 +++++++++++++++++++++++++++++++++++----------
 blockjob.c                 | 185 +++++++++++++++++++----
 docs/bitmaps.md            |   6 +-
 include/block/block.h      |   2 +-
 include/block/block_int.h  |   6 +-
 include/block/blockjob.h   |  83 ++++++++++-
 qapi-schema.json           |  10 +-
 qapi/block-core.json       |  29 ++++
 qmp-commands.hx            |  21 ++-
 tests/Makefile             |   3 +
 tests/qemu-iotests/124     | 182 ++++++++++++++++++++++-
 tests/qemu-iotests/124.out |   4 +-
 tests/test-blockjob-txn.c  | 244 +++++++++++++++++++++++++++++++
 16 files changed, 1057 insertions(+), 143 deletions(-)
 create mode 100644 tests/test-blockjob-txn.c

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2.4.3