diff --git a/diskio.sh b/diskio.sh
index ec1d93f..0757bb8 100755
--- a/diskio.sh
+++ b/diskio.sh
@@ -22,7 +22,9 @@ DESCRIPTION
 
 Icinga plugin to check whether a given path is writable and checks the
 performance of the given path. Detects problems with the filesystem or
-specific io performance.
+specific io performance. Writes a test file using `dd` and reports
+write speed. It is recommended to use a subdirectory tmp for writing
+test files.
 
 EOF
                     exit;;