The FXX is paired with the LXX in that it must have the same physical and logical block size. The number of tracks required is dependent upon the number of tasks generated in the Multi-User Facility or MUFplex.
The FXX is divided into a control block or blocks and log record blocks. Criteria for determining the number of control blocks is based upon the number of total tasks to be supported with this Multi-User Facility or MUFplex. During initialization (INIT AREA=FXX), the TASKS= keyword is used to specify the number of tasks to be supported. See the table that begins on page How to Use for more information on the TASKS= keyword.
The number of control blocks that are reserved is based upon the following formula: 8192 + (tasks * 32). If running the typical 3390 block size of 27998 with less than 618 tasks, all fits in one control block. Nine control blocks would be required with 6999 tasks.
The number of blocks for log records depends upon the number of active requests that could possibly be forced from the LXX and upon the size of the forced log records. Active requests are forced only rarely. However, if the Multi-User Facility requires space and space is not available, all log requests fail until the Multi-User Facility is canceled and restarted. To prevent this situation from occurring, calculate the number of force blocks required to provide space for one log record per task.
An easy approach is to allow one logical block per task. While active requests are unlikely, the following conditions could increase the probability of their occurring:
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