In order to improve the performance and efficient usage of our MARS system the server produces a warning whenever some inefficient access pattern is detected. A typical inefficient request pattern usually involves repeated access to elements of data which are stored in a single tape using individual requests. This pattern forces multiple mounts of a tape (which is a very expensive action for a tape-based mass storage system) which could be avoided if a single combined larger request were to be used.
We could recommend you to read Guidelines to write efficient MARS requests, but please do contact our ServiceDesk via email (servicedesk@ecmwf.int) or our support portal if you need any further advice.