Possibility to physically reserve space without writing leaf blocks

*** 956,965 ****
--- 956,971 ----
  static int
  zio_read_bp_init(zio_t *zio)
  {
          blkptr_t *bp = zio->io_bp;
  
+         if (!BP_IS_EMBEDDED(bp) && BP_GET_PROP_RESERVATION(bp)) {
+                 memset(zio->io_orig_data, 0, zio->io_orig_size);
+                 zio->io_pipeline = ZIO_INTERLOCK_STAGES;
+                 return (ZIO_PIPELINE_CONTINUE);
+         }
+ 
          if (BP_GET_COMPRESS(bp) != ZIO_COMPRESS_OFF &&
              zio->io_child_type == ZIO_CHILD_LOGICAL &&
              !(zio->io_flags & ZIO_FLAG_RAW)) {
                  uint64_t psize =
                      BP_IS_EMBEDDED(bp) ? BPE_GET_PSIZE(bp) : BP_GET_PSIZE(bp);
*** 991,1000 ****
--- 997,1008 ----
  zio_write_bp_init(zio_t *zio)
  {
          spa_t *spa = zio->io_spa;
          zio_prop_t *zp = &zio->io_prop;
          enum zio_compress compress = zp->zp_compress;
+         enum zio_checksum checksum = zp->zp_checksum;
+         uint8_t dedup = zp->zp_dedup;
          blkptr_t *bp = zio->io_bp;
          uint64_t lsize = zio->io_size;
          uint64_t psize = lsize;
          int pass = 1;
  
*** 1009,1018 ****
--- 1017,1032 ----
          if (!IO_IS_ALLOCATING(zio))
                  return (ZIO_PIPELINE_CONTINUE);
  
          ASSERT(zio->io_child_type != ZIO_CHILD_DDT);
  
+         if (zp->zp_zero_write && !(zio->io_pipeline & ZIO_GANG_STAGES)) {
+                 dedup = B_FALSE;
+                 compress = ZIO_COMPRESS_OFF;
+                 checksum = ZIO_CHECKSUM_OFF;
+         }
+ 
          if (zio->io_bp_override) {
                  ASSERT(bp->blk_birth != zio->io_txg);
                  ASSERT(BP_GET_DEDUP(zio->io_bp_override) == 0);
  
                  *bp = *zio->io_bp_override;
*** 1032,1048 ****
                          return (ZIO_PIPELINE_CONTINUE);
                  }
  
                  ASSERT(!zp->zp_nopwrite);
  
!                 if (BP_IS_HOLE(bp) || !zp->zp_dedup)
                          return (ZIO_PIPELINE_CONTINUE);
  
!                 ASSERT(zio_checksum_table[zp->zp_checksum].ci_dedup ||
                      zp->zp_dedup_verify);
  
!                 if (BP_GET_CHECKSUM(bp) == zp->zp_checksum) {
                          BP_SET_DEDUP(bp, 1);
                          zio->io_pipeline |= ZIO_STAGE_DDT_WRITE;
                          return (ZIO_PIPELINE_CONTINUE);
                  }
                  zio->io_bp_override = NULL;
--- 1046,1062 ----
                          return (ZIO_PIPELINE_CONTINUE);
                  }
  
                  ASSERT(!zp->zp_nopwrite);
  
!                 if (BP_IS_HOLE(bp) || !dedup)
                          return (ZIO_PIPELINE_CONTINUE);
  
!                 ASSERT(zio_checksum_table[checksum].ci_dedup ||
                      zp->zp_dedup_verify);
  
!                 if (BP_GET_CHECKSUM(bp) == checksum) {
                          BP_SET_DEDUP(bp, 1);
                          zio->io_pipeline |= ZIO_STAGE_DDT_WRITE;
                          return (ZIO_PIPELINE_CONTINUE);
                  }
                  zio->io_bp_override = NULL;
*** 1151,1160 ****
--- 1165,1185 ----
                  BP_SET_PSIZE(bp, psize);
                  BP_SET_COMPRESS(bp, compress);
                  BP_SET_CHECKSUM(bp, zp->zp_checksum);
                  BP_SET_DEDUP(bp, zp->zp_dedup);
                  BP_SET_BYTEORDER(bp, ZFS_HOST_BYTEORDER);
+                 if (zp->zp_zero_write  && !(zio->io_pipeline & ZIO_GANG_STAGES)) {
+                         boolean_t need_allocate = B_FALSE;
+                         if (zio->io_pipeline & ZIO_STAGE_DVA_ALLOCATE)
+                                 need_allocate = B_TRUE;
+                         zio->io_pipeline = ZIO_INTERLOCK_STAGES;
+                         if (need_allocate)
+                                 zio->io_pipeline |= ZIO_STAGE_DVA_ALLOCATE;
+                         BP_SET_PROP_RESERVATION(bp, 1);
+                 } else {
+                         BP_SET_PROP_RESERVATION(bp, 0);
+                 }
                  if (zp->zp_dedup) {
                          ASSERT(zio->io_child_type == ZIO_CHILD_LOGICAL);
                          ASSERT(!(zio->io_flags & ZIO_FLAG_IO_REWRITE));
                          zio->io_pipeline = ZIO_DDT_WRITE_PIPELINE;
                  }
*** 1869,1878 ****
--- 1894,1904 ----
                  zp.zp_type = DMU_OT_NONE;
                  zp.zp_level = 0;
                  zp.zp_copies = gio->io_prop.zp_copies;
                  zp.zp_dedup = B_FALSE;
                  zp.zp_dedup_verify = B_FALSE;
+                 zp.zp_zero_write = B_FALSE;
                  zp.zp_nopwrite = B_FALSE;
  
                  zio_nowait(zio_write(zio, spa, txg, &gbh->zg_blkptr[g],
                      (char *)pio->io_data + (pio->io_size - resid), lsize, &zp,
                      zio_write_gang_member_ready, NULL, NULL, &gn->gn_child[g],