diff --git a/Optimization.md b/Optimization.md
index 6e94a45..15a0edb 100644
--- a/Optimization.md
+++ b/Optimization.md
@@ -91,9 +91,13 @@ curl "http://localhost:9333/dir/status?pretty=y"
 
 Once you are sure about the volume free spaces, you can use your own file ids. Just need to ensure the file key format is compatible.
 
-The assigned file cookie can also be customized.
+The assigned file cookie can also be customized, as long as it is 32-bit.
 
-Customizing the needle id and/or file cookie is an acceptable behavior. "strict monotonously increasing" is not necessary, but keeping file id in a "mostly" increasing order is expected in order to keep the in memory data structure efficient.
+### Volume Server Efficiency
+
+"strict monotonously increasing" needle id is not necessary, but keeping needle id in a "mostly" increasing order helps to keep the in memory data structure efficient. 
+
+If the needle id have to be random, you can use leveldb as the index when starting volume server.
 
 ## Upload large files