diff --git a/daemon/graphdriver/devmapper/README.md b/daemon/graphdriver/devmapper/README.md
index bf6f2c792c4f76d7d4d7f70e4042d03edd8d64fb..f8eb2faca7fd2cb0c5007ca83bf2130a853572f5 100644
--- a/daemon/graphdriver/devmapper/README.md
+++ b/daemon/graphdriver/devmapper/README.md
@@ -43,7 +43,11 @@ Here is the list of supported options:
     10G. Note, thin devices are inherently "sparse", so a 10G device
     which is mostly empty doesn't use 10 GB of space on the
     pool. However, the filesystem will use more space for the empty
-    case the larger the device is.
+    case the larger the device is. **Warning**: This value affects the
+    system-wide "base" empty filesystem that may already be
+    initialized and inherited by pulled images.  Typically, a change
+    to this value will require additional steps to take effect: 1)
+    stop `docker -d`, 2) `rm -rf /var/lib/docker`, 3) start `docker -d`.
 
     Example use: