--- a/mercurial/revlog.py	Thu Apr 26 03:47:17 2012 +0200
+++ b/mercurial/revlog.py	Fri Apr 27 13:07:29 2012 -0500
@@ -812,7 +812,7 @@
         else:
             df = self.opener(self.datafile)
 
-        readahead = max(_chunksize, length)
+        readahead = max(65536, length)
         df.seek(offset)
         d = df.read(readahead)
         df.close()