JES2 allocates spool space for a job by dividing each spool volume into track groups. A track group consists of one or more DASD tracks. JES2 allocates spool space to a job one track group at a time. When a given track group is exhausted, JES2 allocates the next track group as close to the perimeter of the device as possible on the spool volume. Track groups are obtained from eligible spool volumes on a rotational basis to balance spool I/O. Seek time is therefore minimized.

The TGSIZE= parameter on the SPOOLDEF statement controls the number of JES2 buffers in a track group. JES2 uses this value to calculate the size (in tracks) of a track group for each spool device. Track group size is rounded to the minimum integral number of tracks that can contain TGSIZE buffers (each set to the BUFSIZE= specification on the SPOOLDEF statement). In an environment with mixed spool devices, it is best to specify a value for TGSIZE= that results in consistent spool space allocation for each job regardless of the device type.

Small values for TGSIZE= might cause an excessive number of track group allocations per job. Large values for TGSIZE= might waste spool space and increase seek time. See the descriptions of BUFSIZE=, TGSIZE=, and TGSPACE=(MAX=) parameters on the SPOOLDEF statement in z/OS JES2 Initialization and Tuning Reference for more information on the specification of these parameters, allocation of spool space, and their relation to maximizing device utilization and overall system performance.