diff --git a/mercurial/revlogutils/nodemap.py b/mercurial/revlogutils/nodemap.py
--- a/mercurial/revlogutils/nodemap.py
+++ b/mercurial/revlogutils/nodemap.py
@@ -220,6 +220,11 @@
 
     contains up to 16 entry indexed from 0 to 15"""
 
+    def __init__(self):
+        super(Block, self).__init__()
+        # If this block exist on disk, here is its ID
+        self.ondisk_id = None
+
     def __iter__(self):
         return iter(self.get(i) for i in range(16))
 
@@ -327,8 +332,8 @@
     """Parse the persisted version of an individual block
     """
     block = Block()
-    ondisk_id = len(block_map)
-    block_map[ondisk_id] = block
+    block.ondisk_id = len(block_map)
+    block_map[block.ondisk_id] = block
     values = S_BLOCK.unpack(block_data)
     for idx, v in enumerate(values):
         if v == NO_ENTRY: