[Qemu-devel] [PATCH 0/9] major rework of drive-mirror

["Denis V. Lunev" <den@openvz.org>]

Block commit of the active image to the backing store on a slow disk
could never end. For example with the guest with the following loop
inside
    while true; do
        dd bs=1k count=1 if=/dev/zero of=x
    done
running above slow storage could not complete the operation with a
resonable amount of time:
    virsh blockcommit rhel7 sda --active --shallow
    virsh qemu-monitor-event
    virsh qemu-monitor-command rhel7 \
        '{"execute":"block-job-complete",\
          "arguments":{"device":"drive-scsi0-0-0-0"} }'
    virsh qemu-monitor-event
Completion event is never received.

This problem could not be fixed easily with the current architecture. We
should either prohibit guest writes (making dirty bitmap dirty) or switch
to the sycnchronous scheme.

This series switches driver mirror to synch scheme. Actually we can have
something more intelligent and switch to sync mirroring just after
the first pass over the bitmap. Though this could be done relatively
easily during discussion. The most difficult things are here.

The set also adds some performance improvements dealing with
known-to-be-zero areas.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Vladimir Sementsov-Ogievskiy<vsementsov@virtuozzo.com>
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <famz@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Jeff Cody <jcody@redhat.com>
CC: Eric Blake <eblake@redhat.com>

Denis V. Lunev (9):
  mirror: fix calling of blk_aio_pwritev/blk_aio_preadv
  mirror: create mirror_dirty_init helper for mirror_run
  mirror: optimize dirty bitmap filling in mirror_run a bit
  mirror: efficiently zero out target
  mirror: improve performance of mirroring of empty disk
  block: pass qiov into before_write notifier
  mirror: allow to save buffer for QEMUIOVector in MirrorOp
  mirror: use synch scheme for drive mirror
  mirror: replace bdrv_dirty_bitmap with plain hbitmap

 block/io.c                |  12 +-
 block/mirror.c            | 290 +++++++++++++++++++++++++++++++++++-----------
 include/block/block_int.h |   1 +
 3 files changed, 229 insertions(+), 74 deletions(-)

-- 
2.5.0