Function zio_vdev_child_io

freebsd/contrib/openzfs/module/zfs/zio.c:1416–1489

* Create a child I/O to do some work for us. */

 * Create a child I/O to do some work for us.
 */
zio_t *
zio_vdev_child_io(zio_t *pio, blkptr_t *bp, vdev_t *vd, uint64_t offset,
    abd_t *data, uint64_t size, int type, zio_priority_t priority,
    enum zio_flag flags, zio_done_func_t *done, void *private)
{
	enum zio_stage pipeline = ZIO_VDEV_CHILD_PIPELINE;
	zio_t *zio;

	/*
	 * vdev child I/Os do not propagate their error to the parent.
	 * Therefore, for correct operation the caller *must* check for
	 * and handle the error in the child i/o's done callback.
	 * The only exceptions are i/os that we don't care about
	 * (OPTIONAL or REPAIR).
	 */
	ASSERT((flags & ZIO_FLAG_OPTIONAL) || (flags & ZIO_FLAG_IO_REPAIR) ||
	    done != NULL);

	if (type == ZIO_TYPE_READ && bp != NULL) {
		/*
		 * If we have the bp, then the child should perform the
		 * checksum and the parent need not.  This pushes error
		 * detection as close to the leaves as possible and
		 * eliminates redundant checksums in the interior nodes.
		 */
		pipeline |= ZIO_STAGE_CHECKSUM_VERIFY;
		pio->io_pipeline &= ~ZIO_STAGE_CHECKSUM_VERIFY;
	}

	if (vd->vdev_ops->vdev_op_leaf) {
		ASSERT0(vd->vdev_children);
		offset += VDEV_LABEL_START_SIZE;
	}

	flags |= ZIO_VDEV_CHILD_FLAGS(pio);

	/*
	 * If we've decided to do a repair, the write is not speculative --
	 * even if the original read was.
	 */
	if (flags & ZIO_FLAG_IO_REPAIR)
		flags &= ~ZIO_FLAG_SPECULATIVE;

	/*
	 * If we're creating a child I/O that is not associated with a
	 * top-level vdev, then the child zio is not an allocating I/O.
	 * If this is a retried I/O then we ignore it since we will
	 * have already processed the original allocating I/O.
	 */
	if (flags & ZIO_FLAG_IO_ALLOCATING &&
	    (vd != vd->vdev_top || (flags & ZIO_FLAG_IO_RETRY))) {
		ASSERT(pio->io_metaslab_class != NULL);
		ASSERT(pio->io_metaslab_class->mc_alloc_throttle_enabled);
		ASSERT(type == ZIO_TYPE_WRITE);
		ASSERT(priority == ZIO_PRIORITY_ASYNC_WRITE);
		ASSERT(!(flags & ZIO_FLAG_IO_REPAIR));
		ASSERT(!(pio->io_flags & ZIO_FLAG_IO_REWRITE) ||
		    pio->io_child_type == ZIO_CHILD_GANG);