Update lustre.md

added note about workaround for users to get filesize

doc.zih.tu-dresden.de/docs/data_lifecycle/lustre.md

@@ -46,6 +46,12 @@ Lustre offers a number of commands that are suited to its architecture.
 | `ls -l <filename>` | `ls -l` |
 | `ls` | `ls --color` |
 
+In case commands such as `du` are needed, for example to identify large
+directories, these commands should be applied to as little data as
+possible. You should not just query the main directory in general, you
+should try to work in the sub directories first. The deeper in the
+structure, the better.
+
 ## Useful Commands for Lustre
 
 These commands work for Lustre filesystems `/scratch` and `/ssd`.