dm thin: update Documentation to clarify when "read_only" is valid

Due to user confusion, clarify that it doesn't make sense to try to
create a thin-pool with "read_only" mode enabled.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --git a/Documentation/device-mapper/thin-provisioning.txt b/Documentation/device-mapper/thin-provisioning.txt
index 4bcd4b7..3d01948 100644 (file)
--- a/Documentation/device-mapper/thin-provisioning.txt
+++ b/Documentation/device-mapper/thin-provisioning.txt
@@ -264,7 +264,10 @@ i) Constructor
                           data device, but just remove the mapping.
 
       read_only: Don't allow any changes to be made to the pool
-                metadata.
+                metadata.  This mode is only available after the
+                thin-pool has been created and first used in full
+                read/write mode.  It cannot be specified on initial
+                thin-pool creation.
 
       error_if_no_space: Error IOs, instead of queueing, if no space.