diff --git a/edenscm/hgext/remotefilelog/basepack.py b/edenscm/hgext/remotefilelog/basepack.py
index c9b8218250..968b75c43d 100644
--- a/edenscm/hgext/remotefilelog/basepack.py
+++ b/edenscm/hgext/remotefilelog/basepack.py
@@ -524,6 +524,7 @@ class mutablebasepack(versionmixin):
 
         self._ui = ui
         self._packdir = packdir
+        self._datalen = 0
 
         self.entries = {}
 
@@ -573,7 +574,10 @@ class mutablebasepack(versionmixin):
         self._cleantemppacks()
 
     def writeraw(self, data):
+        self.packfp.seek(self._datalen, os.SEEK_SET)
+        assert self.packfp.tell() == self._datalen
         self.packfp.write(data)
+        self._datalen += len(data)
         self.sha.update(data)
 
     def close(self):
diff --git a/edenscm/hgext/remotefilelog/datapack.py b/edenscm/hgext/remotefilelog/datapack.py
index e230379357..9a8cfe7ed7 100644
--- a/edenscm/hgext/remotefilelog/datapack.py
+++ b/edenscm/hgext/remotefilelog/datapack.py
@@ -542,7 +542,7 @@ class mutabledatapack(basepack.mutablebasepack):
             if metadata and metadata.get(constants.METAKEYFLAG, 0) != 0:
                 raise error.ProgrammingError("v0 pack cannot store flags")
 
-        offset = self.packfp.tell()
+        offset = self._datalen
 
         size = len(rawdata)
 
diff --git a/edenscm/hgext/remotefilelog/historypack.py b/edenscm/hgext/remotefilelog/historypack.py
index e877c13698..89084d463b 100644
--- a/edenscm/hgext/remotefilelog/historypack.py
+++ b/edenscm/hgext/remotefilelog/historypack.py
@@ -463,7 +463,7 @@ class mutablehistorypack(basepack.mutablebasepack):
     def _write(self):
         for filename in sorted(self.fileentries):
             entrymap = self.fileentries[filename]
-            sectionstart = self.packfp.tell()
+            sectionstart = self._datalen
 
             # Write the file section content
             def parentfunc(node):