Computer grids are often used for conducting intensive scientific or technological calculations. Those calculations often require a specific mechanism for manipulating data. Besides demands for data manipulation built in the executions there is also the demand for storing data about data -location of data, accessibility and so on. This data, called metadata is also managed by the grids data management system. In this paper we give an overview of the grid data management problem. We then explain

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... the demands specific for our CRO-GRID. Based on those specific demands we describe a system for data management which brings certain new concepts, such as combining forward reservation and dynamical replication.