* Denis V. Lunev (d...@openvz.org) wrote:
> 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.
Some random thoughts:
a) People have been asking for post-copy for block storage, and this
solves the same problem a different way.
b) I guess the other way is to turn on write-throttling - that would
also guarantee the rate at which writes happen.
c) Don't you end up with something verymuch like what the colo block
replication
guys have ended up with?
Dave
> The set also adds some performance improvements dealing with
> known-to-be-zero areas.
>
> Signed-off-by: Denis V. Lunev <d...@openvz.org>
> Reviewed-by: Vladimir Sementsov-Ogievskiy<vsement...@virtuozzo.com>
> CC: Stefan Hajnoczi <stefa...@redhat.com>
> CC: Fam Zheng <f...@redhat.com>
> CC: Kevin Wolf <kw...@redhat.com>
> CC: Max Reitz <mre...@redhat.com>
> CC: Jeff Cody <jc...@redhat.com>
> CC: Eric Blake <ebl...@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
>
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
