Determining Track Group Allocation Sizes

When a job needs spool space for the first time, JES3 allocates one or more track groups to the job. You can specify how many track groups JES3 should allocate by using the TRKGRPS parameter on the MAINPROC, CLASS, and SYSOUT initialization statements. The person submitting a job can specify the track group allocation size on the //*MAIN JES3 control statement.

When a job uses up its first, or “primary”, allocation of spool space, JES3 allocates more track groups. You can also specify, using the TRKGRPS parameter, how many units JES3 may allocate for each additional, or “secondary”, allocation. JES3 continues allocating secondary quantities of spool space until the job needs no more space.

To determine the track group allocation size, then, you determine two things: how much spool space JES3 should allocate to a job initially, and how much JES3 should allocate thereafter.

Primary TRKGRPS allocation. Use the following general equation to determine the value for your primary TRKGRPS allocation:
```
spool space required for
 average job (in bytes)           1
----------------------   x  -------------  =  primary TRKGRPS value
       BUFSIZE               GRPSZ value
```
You can determine how much spool space an average job in your installation requires by using a resource monitoring program, such as the JES3 Monitoring Facility or the system management facilities (SMF).

Tailor this equation to the particular initialization statement on which you specify the TRKGRPS parameter. For example, if you specify the TRKGRPS parameter on the CLASS statement, use the value for the spool space required for an average job in that job class. If the installation uses spool partitions on a job class basis, use the GRPSZ value from the appropriate SPART statement, not from the BUFFER statement.

Note that the primary TRKGRPS allocation value cannot exceed 9.
Secondary TRKGRPS allocation. The number specified for secondary allocations depends on whether jobs that are larger than average are much larger or only somewhat larger. If the jobs are much larger, you need to specify a larger secondary allocation than if the jobs are only slightly larger. Set the value to keep as low as possible both the number of times JES3 allocates spool space and the amount of unused spool space.
Note that the secondary TRKGRPS allocation value also cannot exceed 9.

Notice that you have greater control over spool space allocation when you use spool partitioning. You can vary track group size, and thus primary and secondary allocation sizes, partition by partition. You can also use spool space more efficiently by assigning large jobs to one spool partition and small jobs to another partition and selecting their GRPSZ and TRKGRPS parameters accordingly.

See the topic entitled Using Spool Partitions for information about defining spool partitions.