Function vdev_queue_io_to_issue

freebsd/contrib/openzfs/module/zfs/vdev_queue.c:863–920

}

static zio_t *
vdev_queue_io_to_issue(vdev_queue_t *vq)
{
	zio_t *zio, *aio;
	zio_priority_t p;
	avl_index_t idx;
	avl_tree_t *tree;

again:
	ASSERT(MUTEX_HELD(&vq->vq_lock));

	p = vdev_queue_class_to_issue(vq);

	if (p == ZIO_PRIORITY_NUM_QUEUEABLE) {
		/* No eligible queued i/os */
		return (NULL);
	}

	/*
	 * For LBA-ordered queues (async / scrub / initializing), issue the
	 * i/o which follows the most recently issued i/o in LBA (offset) order.
	 *
	 * For FIFO queues (sync/trim), issue the i/o with the lowest timestamp.
	 */
	tree = vdev_queue_class_tree(vq, p);
	vq->vq_io_search.io_timestamp = 0;
	vq->vq_io_search.io_offset = vq->vq_last_offset - 1;
	VERIFY3P(avl_find(tree, &vq->vq_io_search, &idx), ==, NULL);
	zio = avl_nearest(tree, idx, AVL_AFTER);
	if (zio == NULL)
		zio = avl_first(tree);
	ASSERT3U(zio->io_priority, ==, p);

	aio = vdev_queue_aggregate(vq, zio);
	if (aio != NULL)
		zio = aio;
	else
		vdev_queue_io_remove(vq, zio);

	/*
	 * If the I/O is or was optional and therefore has no data, we need to
	 * simply discard it. We need to drop the vdev queue's lock to avoid a
	 * deadlock that we could encounter since this I/O will complete
	 * immediately.
	 */
	if (zio->io_flags & ZIO_FLAG_NODATA) {
		mutex_exit(&vq->vq_lock);
		zio_vdev_io_bypass(zio);
		zio_execute(zio);
		mutex_enter(&vq->vq_lock);
		goto again;
	}

	vdev_queue_pending_add(vq, zio);
	vq->vq_last_offset = zio->io_offset + zio->io_size;

	return (zio);
}