Techniques are described for storing portions of files on different storage devices based on the devices' respective storage class. For example, different extents from a large file within a system, application, or database may be stored on different storage devices. The storage devices may be part of a single storage system that includes some storage devices that have different performance capabilities. The decision of where, within the storage classes, to store data may be based on the access patterns of the data. Access patterns regarding file extents are tracked using respective access bits, which are set when a memory block associated with an extent is accessed via an I/O operation. In one embodiment, an access bitmap is modified to account for cached blocks of file extents.