diff --git a/src/otaclient/create_standby/_delta_gen.py b/src/otaclient/create_standby/_delta_gen.py
index 95d37e9c9..30824abcb 100644
--- a/src/otaclient/create_standby/_delta_gen.py
+++ b/src/otaclient/create_standby/_delta_gen.py
@@ -19,12 +19,11 @@
 import logging
 import os
 import threading
-from concurrent.futures import Future, ThreadPoolExecutor
+from concurrent.futures import ThreadPoolExecutor
 from functools import partial
 from hashlib import sha256
 from pathlib import Path
 from queue import Queue
-from typing import Any
 
 from ota_metadata.file_table._orm import (
     FTDirORM,
@@ -53,7 +52,7 @@
 
 SHA256HEXSTRINGLEN = 256 // 8 * 2
 DELETE_BATCH_SIZE = 512
-DB_CONN_NUMS = 3
+DB_CONN_NUMS = 6
 
 
 class DeltaGenerator:
@@ -161,6 +160,7 @@ def _process_file(
         except Exception as e:
             burst_suppressed_logger.exception(f"failed to proces {fpath}: {e!r}")
         finally:
+            self._max_pending_tasks.release()  # always release se first
             tmp_f.unlink(missing_ok=True)
 
     @staticmethod
@@ -168,15 +168,6 @@ def _thread_worker_initializer(thread_local) -> None:
         thread_local.buffer = buffer = bytearray(cfg.CHUNK_SIZE)
         thread_local.view = memoryview(buffer)
 
-    def _task_done_callback(self, _: Future[Any]) -> None:
-        self._max_pending_tasks.release()  # always release se first
-
-    def _check_dir_should_fully_scan(self, dpath: Path) -> bool:
-        for parent in reversed(dpath.parents):
-            if str(parent) in self.FULL_SCAN_PATHS:
-                return True
-        return False
-
     def _post_calculate_delta(self) -> None:
         """
         After all the local resources have been collected, we check the copy_dir
@@ -219,7 +210,7 @@ def _post_calculate_delta(self) -> None:
 
     # API
 
-    def calculate_delta(self) -> None:
+    def calculate_delta(self) -> None:  # NOSONAR
         logger.debug("process delta src, generate delta and prepare local copy...")
         _canonical_root = Path(CANONICAL_ROOT)
 
@@ -247,9 +238,12 @@ def calculate_delta(self) -> None:
                 )
 
                 # ------ check whether we should skip this folder ------ #
-                dir_should_fully_scan = self._check_dir_should_fully_scan(
-                    canonical_curdir_path
-                )
+                dir_should_fully_scan = False
+                for parent in reversed(canonical_curdir_path.parents):
+                    if str(parent) in self.FULL_SCAN_PATHS:
+                        dir_should_fully_scan = True
+                        break
+
                 dir_depth_exceeded = (
                     len(canonical_curdir_path.parents) > self.MAX_FOLDER_DEEPTH
                 )
@@ -258,7 +252,7 @@ def calculate_delta(self) -> None:
                 _should_skip_dir = dir_depth_exceeded or (
                     _str_canon_fpath != CANONICAL_ROOT
                     and not dir_should_fully_scan
-                    and not self._ft_dir_orm.orm_select_entry(path=_str_canon_fpath)
+                    and not self._ft_dir_orm.check_entry(path=_str_canon_fpath)
                 )
                 if _should_skip_dir:
                     dirnames.clear()
@@ -291,7 +285,7 @@ def calculate_delta(self) -> None:
                         canonical_curdir_path / fname,
                         fully_scan=dir_should_fully_scan,
                         thread_local=thread_local,
-                    ).add_done_callback(self._task_done_callback)
+                    )
         finally:
             pool.shutdown(wait=True)
             self._ft_regular_orm.orm_pool_shutdown()