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>

*** 1332,1342 ****
                  return (0);
  
          rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
  
          /* Check for any allocated blocks beyond the first */
!         if (dn->dn_phys->dn_maxblkid != 0)
                  goto fail;
  
          mutex_enter(&dn->dn_dbufs_mtx);
          for (db = list_head(&dn->dn_dbufs); db; db = db_next) {
                  db_next = list_next(&dn->dn_dbufs, db);
--- 1332,1342 ----
                  return (0);
  
          rw_enter(&dn->dn_struct_rwlock, RW_WRITER);
  
          /* Check for any allocated blocks beyond the first */
!         if (dn->dn_maxblkid != 0)
                  goto fail;
  
          mutex_enter(&dn->dn_dbufs_mtx);
          for (db = list_head(&dn->dn_dbufs); db; db = db_next) {
                  db_next = list_next(&dn->dn_dbufs, db);