block/mirror: Do not wait for active writes

block/mirror: Do not wait for active writes

Waiting for all active writes to settle before daring to create a
background copying operation means that we will never do background
operations while the guest does anything (in write-blocking mode), and
therefore cannot converge.  Yes, we also will not diverge, but actually
converging would be even nicer.

It is unclear why we did decide to wait for all active writes to settle
before creating a background operation, but it just does not seem
necessary.  Active writes will put themselves into the in_flight bitmap
and thus properly block actually conflicting background requests.

It is important for active requests to wait on overlapping background
requests, which we do in active_write_prepare().  However, so far it was
not documented why it is important.  Add such documentation now, and
also to the other call of mirror_wait_on_conflicts(), so that it becomes
more clear why and when requests need to actively wait for other
requests to settle.

Another thing to note is that of course we need to ensure that there are
no active requests when the job completes, but that is done by virtue of
the BDS being drained anyway, so there cannot be any active requests at
that point.

With this change, we will need to explicitly keep track of how many
bytes are in flight in active requests so that
job_progress_set_remaining() in mirror_run() can set the correct number
of remaining bytes.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2123297
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20221109165452.67927-2-hreitz@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>