diff --git a/Customization.md b/Customization.md
index 93c9acf..317374c 100644
--- a/Customization.md
+++ b/Customization.md
@@ -122,6 +122,20 @@ When `ccls` indexes `/tmp/host/proj/a.cc`, the cache file
 (re-indexed), the newly generated `a.cc.blob` will not contain
 `/tmp/container` paths any more.
 
+#### `cache.retainInMemory`
+
+Default: 1
+
+Change to 0 if you want to save memory, but having multiple ccls processes operating in the same directory may corrupt ccls's in-memory representation of the index.
+
+After this number of loads, keep a copy of file index in memory (which
+increases memory usage). During incremental updates, the removed file
+index will be taken from the in-memory copy, instead of the on-disk file.
+
+Every index action is counted: the initial load, a save action.
+0: never retain; 1: retain after initial load; 2: retain after 2 loads
+(initial load+first save)
+
 #### `clang.resourceDir`
 
 Default: _empty_
diff --git a/Debugging.md b/Debugging.md
index 0e9f6b0..72473e6 100644
--- a/Debugging.md
+++ b/Debugging.md
@@ -6,6 +6,8 @@ First of all, use a [`Debug` build](Build#cmake-options). It is compiled with
 Enable [logging](Customization#--log-filefile----log-file-appendfile) and
 [verbose](Customization#-vn) mode when starting the server.
 
+`-log-file=/tmp/ccls.log -v=1`
+
 ### stderr
 
 You can find stderr output in: