A data storage library system, preferably arranged in an array of independent libraries, uses a log-structured file (LSF) data architecture and assigned roles for the individual storage devices in the library. Each library includes a plurality of storage devices, such as optical disk drives, and a store of removable media units, such as optical disks, that are mounted and dismounted from the storage devices. The log-structured library is similar to fixed media systems having LSF data architectures in that it employs a directory to map the local address of a data set to its physical storage location, but is different in that it divides the jobs of reading and writing (log accesses) and garbage collection among the different storage devices of the library. As required, the dismountable media units are moved from the storage area and mounted on a device assigned the appropriate role. The roles assigned to the storage devices are flexible and can change as needed. The system allows both log accesses and garbage collection to occur simultaneously. The garbage collection process segregates the data into "hot" and "cold" segments, according to the age of the data. This allows all the cold data to be located on the same media units, which can then be exported from the library for off-line storage and replacement with new media units.