(GSS) A disk scheduling strategy in which requests are served in cycles, in a round-robin manner. To reduce disk arm movements ("seeking"), the set of streams is divided into groups that are served in fixed order. Streams within a group are served according to "SCAN".
If all clients are assigned to one group, GSS reduces to SCAN, and if all clients are assigned to separate groups, GSS effectively becomes round-robin scheduling. The service order within one group is not fixed, and a stream may in fact be first in one cycle while last in the next. This variation has to be masked by extra buffering but whereas SCAN requires buffer space for all streams, GSS can reuse the buffer for each group and effect a trade-off between seek optimisation and buffer requirements.