A scalable approach is disclosed for processing auxiliary-copy jobs in a storage management system by using distributed media agent resources instead of a centralized storage manager. Enhanced media agents coordinate and control auxiliary-copy jobs and tap the storage manager to reserve data streams and provide job-specific metadata on demand. An enhanced storage manager may initially select a media agent as “coordinator” to coordinate auxiliary-copy jobs with any number of other media agents, which act as “controllers.” A coordinator media agent is generally responsible for obtaining data stream reservation information from the storage manager and assigning auxiliary-copy jobs to respective controller media agents, based on the components involved in the respective reserved data streams.