[Qemu-block] [PATCH v3 0/2] Fix slow startup with many disks

[Alexander Yarygin]

Changes in v3:
   - Added aio_context_acquire/aio_context_release around blk_drain() in
     "virtio-blk: Use blk_drain() to drain IO requests" + updated commit
     description

Please update Cc: qemu-stable@ if it necessarily.

Changes in v2:
    - Patch "block-backend: Introduce blk_drain() and replace blk_drain_all()"
      splitted to two.
    - blk_drain() moved to above virtio_blk_data_plane_stop().

Previous thread:
http://lists.gnu.org/archive/html/qemu-devel/2015-06/msg02786.html

During reset the aio_poll() function is called at least amount_of_disks^2 times:

for_each disk
    virtio_blk_reset()
        bdrv_drain_all()
                for_each disk
                         aio_poll()

For example, startup with 1000 disks takes over 13 minutes.

Patches 1 and 2 removes inner loop by using bdrv_drain() instead
of bdrv_drain_all(). bdrv_drain() works on one disk at time.

Since bdrv_drain_all() is still called in other places, patch 3 optimizes
it for cases, where there are more disks than iothreads.

Thanks.

Alexander Yarygin (2):
  block-backend: Introduce blk_drain()
  virtio-blk: Use blk_drain() to drain IO requests

 block/block-backend.c          |  5 +++++
 hw/block/virtio-blk.c          | 15 ++++++++++-----
 include/sysemu/block-backend.h |  1 +
 3 files changed, 16 insertions(+), 5 deletions(-)

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1.9.1