Add a blurb about /common's compression capability and reported space.

content/guides/handling_data/using_the_common_file_system.md

@@ -65,4 +65,23 @@ to quickly do maintenance on a single cluster without having to unmount
         for each cluster.
     -   If you use  `module` things should be just fine!
 
+---
+### /common and used space reporting ###
+The /common file system has the capability to compress files so they store
+less data on the underlying disk storage.  Tools like `du` will report the
+true amount of space consumed by files by default.  If the files have been
+compressed before being stored to disk, the report will appear smaller
+than what may be expected.  Passing the `--apparent-size` argument to
+`du` will cause the report to be the uncompressed size of consumed space.
 
+{{< highlight bash >}}
+$ pwd
+/common/demo/demo01
+$ python -c 'print "Hello World!\n" * 2**20,' > hello_world.txt
+$ ls -lh hello_world.txt 
+-rw-r--r-- 1 demo01 demo 13M Mar  7 12:55 hello_world.txt
+$ du -sh hello_world.txt 
+2.0K    hello_world.txt
+$ du -sh --apparent-size hello_world.txt 
+13M     hello_world.txt
+{{< /highlight >}}