[PATCH v2(RFC) 0/3] qcow2: fix parallel rewrite and discard

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Hi all! It occurs that nothing prevents discarding and reallocating host
cluster during data writing. This way data writing will pollute another
newly allocated cluster of data or metadata.

OK, v2 is a try to solve the problem with CoRwlock.. And it is marked
RFC, because of a lot of iotest failures.. Some of problems with v2:

1. It's a more complicated to make a test, as everything is blocking
and I can't just break write and do discard.. I have to implement
aio_discard in qemu-io and rewrite test into several portions of io
commands splitted by "sleep 1".. OK, it's not a big problem, and I've
solved it.

2. iotest 7 fails with several leaked clusters. Seems, that it depend on
the fact that discard may be done in parallel with writes. Iotest 7 does
snapshots, so I think l1 table is updated to the moment when discard is
finally unlocked.. But I didn't dig into it, it's all my assumptions.

3. iotest 13 (and I think a lot more iotests) crashes on
assert(!to->locks_held); .. So with this assertion we can't keep rwlock
locked during data writing...

  #3  in __assert_fail () from /lib64/libc.so.6
  #4  in qemu_aio_coroutine_enter (ctx=0x55762120b700, co=0x55762121d700)
      at ../util/qemu-coroutine.c:158
  #5  in aio_co_enter (ctx=0x55762120b700, co=0x55762121d700) at ../util/async.c:628
  #6  in aio_co_wake (co=0x55762121d700) at ../util/async.c:612
  #7  in thread_pool_co_cb (opaque=0x7f17950daab0, ret=0) at ../util/thread-pool.c:279
  #8  in thread_pool_completion_bh (opaque=0x5576211e5070) at ../util/thread-pool.c:188
  #9  in aio_bh_call (bh=0x557621205df0) at ../util/async.c:136
  #10 in aio_bh_poll (ctx=0x55762120b700) at ../util/async.c:164
  #11 in aio_poll (ctx=0x55762120b700, blocking=true) at ../util/aio-posix.c:659
  #12 in blk_prw (blk=0x557621205790, offset=4303351808, 
      buf=0x55762123e000 '\364' <repeats 199 times>, <incomplete sequence \364>..., bytes=12288, 
      co_entry=0x557620d9dc97 <blk_write_entry>, flags=0) at ../block/block-backend.c:1335
  #13 in blk_pwrite (blk=0x557621205790, offset=4303351808, buf=0x55762123e000, 
      count=12288, flags=0) at ../block/block-backend.c:1501


So now I think that v1 is simpler.. It's more complicated (but not too
much) in code. But it keeps discards and data writes non-blocking each
other and avoids yields in critical sections.

Vladimir Sementsov-Ogievskiy (3):
  qemu-io: add aio_discard
  iotests: add qcow2-discard-during-rewrite
  block/qcow2: introduce inflight writes counters: fix discard

 block/qcow2.h                                 |   2 +
 block/qcow2-cluster.c                         |   4 +
 block/qcow2.c                                 |  18 ++-
 qemu-io-cmds.c                                | 117 ++++++++++++++++++
 .../tests/qcow2-discard-during-rewrite        |  99 +++++++++++++++
 .../tests/qcow2-discard-during-rewrite.out    |  17 +++
 6 files changed, 256 insertions(+), 1 deletion(-)
 create mode 100755 tests/qemu-iotests/tests/qcow2-discard-during-rewrite
 create mode 100644 tests/qemu-iotests/tests/qcow2-discard-during-rewrite.out

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2.29.2