A system (100, 300) like a video on demand server has a storage medium (106) with a number of files (200) from which data can be fetched or in which data can be stored. A file contains a number of clusters (202, 204, 206) and each cluster contains a number of storage sectors (208) that are physically successively organized on the storage medium. When a new stream is requested, either for reading or writing, a block size (210) is determined for the new stream. The block size is large enough to support the rate of the stream and is chosen such that a cluster can be read with an integer number of blocks by evenly partitioning the size of the cluster into the size of the blocks reading the cluster.