diff --git a/FAQ.md b/FAQ.md
index d18f5a2..44c4cfe 100644
--- a/FAQ.md
+++ b/FAQ.md
@@ -18,6 +18,8 @@ Specifically,
 
 The default volume size is 30GB, and default to 8 volumes. Very likely you would need to customize a bit. Either through reducing the volume size in `weed master -volumeSizeLimitMB=xxx`, or increasing the `weed volume -max=xxx` value.
 
+`-volumePreallocate` will preallocate disk spaces to volumes. These disk spaces are "taken" by the assigned volume, and could not be used by other volumes, even though the "df" command would report them as "free" if the volumes are not full.
+
 ### How to access the server dashboard?
 SeaweedFS has web dashboards for its different services: