diff --git a/linux/HardDiskCaching.sls b/linux/HardDiskCaching.sls
index 4fbcd35..4d3a333 100644
--- a/linux/HardDiskCaching.sls
+++ b/linux/HardDiskCaching.sls
@@ -1,3 +1,4 @@
+# https://lonesysadmin.net/2013/12/22/better-linux-disk-caching-performance-vm-dirty_ratio/
 {% if grains['kernel'] == 'Linux' %}
   vm.dirty_background_ratio:
     sysctl.present: