On Mon, Jan 22, 2018 at 05:45:49PM +0300, Vladimir Sementsov-Ogievskiy wrote: > Is it a bug or a feature? Why do we call inc/dec twice for read/write? > We don't do this for flush and discard..
It's non-obvious and I asked Paolo the same question previously.
> - bdrv_inc_in_flight(bs);
> -
> /* throttling disk I/O */
> if (blk->public.throttle_group_member.throttle_state) {
>
> throttle_group_co_io_limits_intercept(&blk->public.throttle_group_member,
> bytes, false);
> }
^^^ HINT HINT HINT ^^^
>
> - ret = bdrv_co_preadv(blk->root, offset, bytes, qiov, flags);
> - bdrv_dec_in_flight(bs);
> - return ret;
> + return bdrv_co_preadv(blk->root, offset, bytes, qiov, flags);
The problem is what happens if the request is throttled?
Even throttled requests must be counted so that bdrv_drain() and friends
work.
It may be possible to eliminate this now that throttling is a BDS node.
It used to be implemented as a completely separate API outside the BDS
node graph. Now there is a throttling node in the graph, so maybe we
can stop taking the extra reference but some refactoring may be
necessary.
Stefan
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