Possibility to physically reserve space without writing leaf blocks

*** 1019,1029 ****
  
          (void) arc_release(db->db_buf, db);
  }
  
  dbuf_dirty_record_t *
! dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
  {
          dnode_t *dn;
          objset_t *os;
          dbuf_dirty_record_t **drp, *dr;
          int drop_struct_lock = FALSE;
--- 1019,1037 ----
  
          (void) arc_release(db->db_buf, db);
  }
  
  dbuf_dirty_record_t *
! dbuf_zero_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx)
! {
!         ASSERT(db->db_objset != NULL);
! 
!         return (dbuf_dirty(db, tx, B_TRUE));
! }
! 
! dbuf_dirty_record_t *
! dbuf_dirty(dmu_buf_impl_t *db, dmu_tx_t *tx, boolean_t zero_write)
  {
          dnode_t *dn;
          objset_t *os;
          dbuf_dirty_record_t **drp, *dr;
          int drop_struct_lock = FALSE;
*** 1150,1159 ****
--- 1158,1168 ----
           * If this buffer is dirty in an old transaction group we need
           * to make a copy of it so that the changes we make in this
           * transaction group won't leak out when we sync the older txg.
           */
          dr = kmem_zalloc(sizeof (dbuf_dirty_record_t), KM_SLEEP);
+         dr->dr_zero_write = zero_write;
          if (db->db_level == 0) {
                  void *data_old = db->db_buf;
  
                  if (db->db_state != DB_NOFILL) {
                          if (db->db_blkid == DMU_BONUS_BLKID) {
*** 1263,1273 ****
                          parent_held = TRUE;
                  }
                  if (drop_struct_lock)
                          rw_exit(&dn->dn_struct_rwlock);
                  ASSERT3U(db->db_level+1, ==, parent->db_level);
!                 di = dbuf_dirty(parent, tx);
                  if (parent_held)
                          dbuf_rele(parent, FTAG);
  
                  mutex_enter(&db->db_mtx);
                  /*
--- 1272,1282 ----
                          parent_held = TRUE;
                  }
                  if (drop_struct_lock)
                          rw_exit(&dn->dn_struct_rwlock);
                  ASSERT3U(db->db_level+1, ==, parent->db_level);
!                 di = dbuf_dirty(parent, tx, B_FALSE);
                  if (parent_held)
                          dbuf_rele(parent, FTAG);
  
                  mutex_enter(&db->db_mtx);
                  /*
*** 1406,1416 ****
          DB_DNODE_ENTER(db);
          if (RW_WRITE_HELD(&DB_DNODE(db)->dn_struct_rwlock))
                  rf |= DB_RF_HAVESTRUCT;
          DB_DNODE_EXIT(db);
          (void) dbuf_read(db, NULL, rf);
!         (void) dbuf_dirty(db, tx);
  }
  
  void
  dmu_buf_will_not_fill(dmu_buf_t *db_fake, dmu_tx_t *tx)
  {
--- 1415,1425 ----
          DB_DNODE_ENTER(db);
          if (RW_WRITE_HELD(&DB_DNODE(db)->dn_struct_rwlock))
                  rf |= DB_RF_HAVESTRUCT;
          DB_DNODE_EXIT(db);
          (void) dbuf_read(db, NULL, rf);
!         (void) dbuf_dirty(db, tx, B_FALSE);
  }
  
  void
  dmu_buf_will_not_fill(dmu_buf_t *db_fake, dmu_tx_t *tx)
  {
*** 1433,1443 ****
  
          ASSERT(db->db.db_object != DMU_META_DNODE_OBJECT ||
              dmu_tx_private_ok(tx));
  
          dbuf_noread(db);
!         (void) dbuf_dirty(db, tx);
  }
  
  #pragma weak dmu_buf_fill_done = dbuf_fill_done
  /* ARGSUSED */
  void
--- 1442,1470 ----
  
          ASSERT(db->db.db_object != DMU_META_DNODE_OBJECT ||
              dmu_tx_private_ok(tx));
  
          dbuf_noread(db);
!         (void) dbuf_dirty(db, tx, B_FALSE);
! }
! 
! 
! void
! dmu_buf_will_zero_fill(dmu_buf_t *db_fake, dmu_tx_t *tx)
! {
!         dmu_buf_impl_t *db = (dmu_buf_impl_t *)db_fake;
! 
!         ASSERT(db->db_blkid != DMU_BONUS_BLKID);
!         ASSERT(tx->tx_txg != 0);
!         ASSERT(db->db_level == 0);
!         ASSERT(!refcount_is_zero(&db->db_holds));
! 
!         ASSERT(db->db.db_object != DMU_META_DNODE_OBJECT ||
!             dmu_tx_private_ok(tx));
! 
!         dbuf_noread(db);
!         (void) dbuf_zero_dirty(db, tx);
  }
  
  #pragma weak dmu_buf_fill_done = dbuf_fill_done
  /* ARGSUSED */
  void
*** 1518,1528 ****
          ASSERT(db->db_state == DB_CACHED || db->db_state == DB_UNCACHED);
  
          if (db->db_state == DB_CACHED &&
              refcount_count(&db->db_holds) - 1 > db->db_dirtycnt) {
                  mutex_exit(&db->db_mtx);
!                 (void) dbuf_dirty(db, tx);
                  bcopy(buf->b_data, db->db.db_data, db->db.db_size);
                  VERIFY(arc_buf_remove_ref(buf, db));
                  xuio_stat_wbuf_copied();
                  return;
          }
--- 1545,1555 ----
          ASSERT(db->db_state == DB_CACHED || db->db_state == DB_UNCACHED);
  
          if (db->db_state == DB_CACHED &&
              refcount_count(&db->db_holds) - 1 > db->db_dirtycnt) {
                  mutex_exit(&db->db_mtx);
!                 (void) dbuf_dirty(db, tx, B_FALSE);
                  bcopy(buf->b_data, db->db.db_data, db->db.db_size);
                  VERIFY(arc_buf_remove_ref(buf, db));
                  xuio_stat_wbuf_copied();
                  return;
          }
*** 1549,1559 ****
          }
          ASSERT(db->db_buf == NULL);
          dbuf_set_data(db, buf);
          db->db_state = DB_FILL;
          mutex_exit(&db->db_mtx);
!         (void) dbuf_dirty(db, tx);
          dmu_buf_fill_done(&db->db, tx);
  }
  
  /*
   * "Clear" the contents of this dbuf.  This will mark the dbuf
--- 1576,1586 ----
          }
          ASSERT(db->db_buf == NULL);
          dbuf_set_data(db, buf);
          db->db_state = DB_FILL;
          mutex_exit(&db->db_mtx);
!         (void) dbuf_dirty(db, tx, B_FALSE);
          dmu_buf_fill_done(&db->db, tx);
  }
  
  /*
   * "Clear" the contents of this dbuf.  This will mark the dbuf
*** 2818,2827 ****
--- 2845,2864 ----
          wp_flag |= (db->db_state == DB_NOFILL) ? WP_NOFILL : 0;
  
          dmu_write_policy(os, dn, db->db_level, wp_flag, &zp);
          DB_DNODE_EXIT(db);
  
+         if (dr->dr_zero_write) {
+                 zp.zp_zero_write = B_TRUE;
+ 
+                 if (!spa_feature_is_active(os->os_spa, SPA_FEATURE_SPACE_RESERVATION))
+                 {
+                         spa_feature_incr(os->os_spa,
+                             SPA_FEATURE_SPACE_RESERVATION, tx);
+                 }
+         }
+ 
          if (db->db_level == 0 &&
              dr->dt.dl.dr_override_state == DR_OVERRIDDEN) {
                  /*
                   * The BP for this block has been provided by open context
                   * (by dmu_sync() or dmu_buf_write_embedded()).