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As the CDS is a distributed platform, CDS datasets are hosted at a number of different locations. Some data are stored on the CDS disks; others are hosted on the data providers' storage systems, e.g. the ECMWF Meteorological Archival and Retrieval System (MARS).
Table 2 presents the ERA5 datasets which are available on CDS disks ('MARS internal)'. Table 3 and Table 4 show the CDS datasets which are stored in the tape library (referred to as 'MARS external'), and in the data providers archives respectively.
The way the user selects the items to include in a single CDS request can make a significant difference in terms of performance. Please see the page Efficiency tips for how to build an efficient request.
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Moreover, CDS data requests also have limits in terms of number of fields and volume size which are different for each dataset (see the the tables below). These values are dependent on each dataset's structure and where the data is stored. Again, these restrictions are introduced to help the system maintain good performance and minimise the queue time.
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