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The MARS system responds to tens of thousands of requests daily. The duration of a retrieval depends on the number of concurrent requests the system is processing and the amount of data retrieved, as well as the number of tape mounts involved. You can monitor the progress of your requests from the Server Activity.
Another useful tool is the Parameter Database which for each parameter shows how it is coded in GRIB format and lists the parameter ID, long and short name and the unit. For many parameters it will also provide links to the IFS documentation.