Updated FAQ (markdown)

FAQ.md

@@ -80,6 +80,23 @@ The volumes are consistent, but not necessarily the same size or the same number
 * If some files are written only to some but not all of the replicas, the writes are considered failed (A best-effort attempt will try to delete the written files).
 * The compaction may not happen at exactly the same time.
 
+### Why files are deleted by disk spaces are not released?
+
+The disk spaces are released when volume is vacuumed. By default, the vacuum only happens when garbage is more than 30%.
+
+You can use `weed shell` to run `volume.vacuum -garbageThreshold=0.0001` to trigger the vacuum.
+
+```
+$ weed shell
+master: localhost:9333 filer: localhost:8888
+> lock
+>  volume.vacuum -h
+Usage of volume.vacuum:
+  -garbageThreshold float
+    	vacuum when garbage is more than this limit (default 0.3)
+>
+```
+
 ### How to store large logs?
 
 The log files are usually very large. Use `weed filer` to store them. 
@@ -122,5 +139,5 @@ The last line shows the folder that already mounted something. Need to unmount i
 That should be it!
 
 ## Upgrade
-### How to upgrade from release 1.xx to 1.yy
+### How to upgrade from release 1.xx to a.yy
 Unless special notes, all upgrade will be backward compatible and ensure no data loss. There could be CLI tweaks. But in general, just test the CLI and make sure it can run.