4101 metaslab_debug should allow for fine-grained control
4102 space_maps should store more information about themselves
4103 space map object blocksize should be increased
4104 ::spa_space no longer works
4105 removing a mirrored log device results in a leaked object
4106 asynchronously load metaslab
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Sebastien Roy <seb@delphix.com>

*** 281,293 ****
                  if (vd->vdev_removing)
                          fnvlist_add_uint64(nv, ZPOOL_CONFIG_REMOVING,
                              vd->vdev_removing);
          }
  
!         if (vd->vdev_dtl_smo.smo_object != 0)
                  fnvlist_add_uint64(nv, ZPOOL_CONFIG_DTL,
!                     vd->vdev_dtl_smo.smo_object);
  
          if (vd->vdev_crtxg)
                  fnvlist_add_uint64(nv, ZPOOL_CONFIG_CREATE_TXG, vd->vdev_crtxg);
  
          if (getstats) {
--- 281,294 ----
                  if (vd->vdev_removing)
                          fnvlist_add_uint64(nv, ZPOOL_CONFIG_REMOVING,
                              vd->vdev_removing);
          }
  
!         if (vd->vdev_dtl_sm != NULL) {
                  fnvlist_add_uint64(nv, ZPOOL_CONFIG_DTL,
!                     space_map_object(vd->vdev_dtl_sm));
!         }
  
          if (vd->vdev_crtxg)
                  fnvlist_add_uint64(nv, ZPOOL_CONFIG_CREATE_TXG, vd->vdev_crtxg);
  
          if (getstats) {