Function vdev_queue_aggregate

freebsd/contrib/openzfs/module/zfs/vdev_queue.c:645–861

* Sufficiently adjacent io_offset's in ZIOs will be aggregated. We do this * by creating a gang ABD from the adjacent ZIOs io_abd's. By using * a gang ABD we avoid doing memory copies to and from the parent, * child ZIOs. The gang ABD also accounts for gaps between adjacent * io_offsets by simply getting the zero ABD for writes or allocating * a new ABD for reads and placing them in the gang

 * a new ABD for reads and placing them in the gang ABD as well.
 */
static zio_t *
vdev_queue_aggregate(vdev_queue_t *vq, zio_t *zio)
{
	zio_t *first, *last, *aio, *dio, *mandatory, *nio;
	zio_link_t *zl = NULL;
	uint64_t maxgap = 0;
	uint64_t size;
	uint64_t limit;
	int maxblocksize;
	boolean_t stretch = B_FALSE;
	avl_tree_t *t = vdev_queue_type_tree(vq, zio->io_type);
	enum zio_flag flags = zio->io_flags & ZIO_FLAG_AGG_INHERIT;
	uint64_t next_offset;
	abd_t *abd;

	maxblocksize = spa_maxblocksize(vq->vq_vdev->vdev_spa);
	if (vq->vq_vdev->vdev_nonrot)
		limit = zfs_vdev_aggregation_limit_non_rotating;
	else
		limit = zfs_vdev_aggregation_limit;
	limit = MAX(MIN(limit, maxblocksize), 0);

	if (zio->io_flags & ZIO_FLAG_DONT_AGGREGATE || limit == 0)
		return (NULL);

	/*
	 * While TRIM commands could be aggregated based on offset this
	 * behavior is disabled until it's determined to be beneficial.
	 */
	if (zio->io_type == ZIO_TYPE_TRIM && !zfs_vdev_aggregate_trim)
		return (NULL);

	/*
	 * I/Os to distributed spares are directly dispatched to the dRAID
	 * leaf vdevs for aggregation.  See the comment at the end of the
	 * zio_vdev_io_start() function.
	 */
	ASSERT(vq->vq_vdev->vdev_ops != &vdev_draid_spare_ops);

	first = last = zio;

	if (zio->io_type == ZIO_TYPE_READ)
		maxgap = zfs_vdev_read_gap_limit;

	/*
	 * We can aggregate I/Os that are sufficiently adjacent and of
	 * the same flavor, as expressed by the AGG_INHERIT flags.
	 * The latter requirement is necessary so that certain
	 * attributes of the I/O, such as whether it's a normal I/O
	 * or a scrub/resilver, can be preserved in the aggregate.
	 * We can include optional I/Os, but don't allow them
	 * to begin a range as they add no benefit in that situation.
	 */

	/*
	 * We keep track of the last non-optional I/O.
	 */
	mandatory = (first->io_flags & ZIO_FLAG_OPTIONAL) ? NULL : first;