3740 Poor ZFS send / receive performance due to snapshot hold / release processing
Submitted by: Steven Hartland <steven.hartland@multiplay.co.uk>

*** 834,860 ****
  {
          zap_attribute_t za;
          zap_cursor_t zc;
          objset_t *mos = dp->dp_meta_objset;
          uint64_t zapobj = dp->dp_tmp_userrefs_obj;
  
          if (zapobj == 0)
                  return;
          ASSERT(spa_version(dp->dp_spa) >= SPA_VERSION_USERREFS);
  
          for (zap_cursor_init(&zc, mos, zapobj);
              zap_cursor_retrieve(&zc, &za) == 0;
              zap_cursor_advance(&zc)) {
                  char *htag;
                  uint64_t dsobj;
  
                  htag = strchr(za.za_name, '-');
                  *htag = '\0';
                  ++htag;
!                 dsobj = strtonum(za.za_name, NULL);
!                 dsl_dataset_user_release_tmp(dp, dsobj, htag);
          }
          zap_cursor_fini(&zc);
  }
  
  /*
   * Create the pool-wide zap object for storing temporary snapshot holds.
--- 834,872 ----
  {
          zap_attribute_t za;
          zap_cursor_t zc;
          objset_t *mos = dp->dp_meta_objset;
          uint64_t zapobj = dp->dp_tmp_userrefs_obj;
+         nvlist_t *holds;
  
          if (zapobj == 0)
                  return;
          ASSERT(spa_version(dp->dp_spa) >= SPA_VERSION_USERREFS);
  
+         holds = fnvlist_alloc();
+ 
          for (zap_cursor_init(&zc, mos, zapobj);
              zap_cursor_retrieve(&zc, &za) == 0;
              zap_cursor_advance(&zc)) {
                  char *htag;
                  uint64_t dsobj;
+                 nvlist_t *tags;
  
                  htag = strchr(za.za_name, '-');
                  *htag = '\0';
                  ++htag;
!                 if (nvlist_lookup_nvlist(holds, za.za_name, &tags) != 0) {
!                         tags = fnvlist_alloc();
!                         fnvlist_add_boolean(tags, htag);
!                         fnvlist_add_nvlist(holds, za.za_name, tags);
!                         fnvlist_free(tags);
!                 } else {
!                         fnvlist_add_boolean(tags, htag);
!                 }
          }
+         dsl_dataset_user_release_tmp(dp, holds);
+         fnvlist_free(holds);
          zap_cursor_fini(&zc);
  }
  
  /*
   * Create the pool-wide zap object for storing temporary snapshot holds.