Provided are a computer program product, system, and method for data unit classification in accordance with one embodiment of the present description, in which in response to a data processing command, a storage controller classifies data units of a storage unit as either allocated to a data set or as unallocated to any data set. If allocated to a data set, the storage controller can further classify data set-allocated data units as either containing client data or metadata or as empty. In accordance with one aspect of the present description, the storage controller may bypass data processing of the data units which have not been allocated to any data set or otherwise do not contain client data or metadata. Other aspects of data unit classification in accordance with the present description are described.