diff --git a/Initialization-options.md b/Initialization-options.md
index 51ac618..1fbf128 100644
--- a/Initialization-options.md
+++ b/Initialization-options.md
@@ -8,6 +8,8 @@ Each language client has its own way to specify initialization options. ccls sup
 
 It points to a directory (relative or absolute) storing indexing files so that next time ccls can load these indexing results without full indexing.
 
+If empty, cache will be stored in memory.
+
 ## `clang.extraArgs` `clang.excludeArgs`
 
 Additional arguments or excluded arguments for `compile_commands.json` entries.
@@ -56,7 +58,15 @@ You usually want to use `index.multiVersionBlacklist` to exclude system headers.
 It is `false` by default which means to re-index when a document is saved.
 Set it to `true` to re-index for every change to a document.
 
-The performance will suffer a lot.
+The performance may suffer a lot.
+
+This is best used in conjunction with empty empty `cacheDirectory` to avoid writing cache files:
+```javascript
+{
+  "cacheDirectory": "",
+  "index": {"onChange": true}
+}
+```
 
 ## `cacheFormat`: serialization format of cache files