Both OS/390 and MSO place restrictions on ddnames.
OS/390 restricts the storage allocated to the Task Input/Output Table (TIOT), which contains data associated with ddnames, to 32K bytes. As each TIOT entry is 20 bytes, there is a limit of about 1600 total allocations possible in an MSO address space. It is possible to increase the size of the TIOT to a maximum of 64K to give a total of about 3200 total allocations. If an MSO address space is anticipated to require this many allocations, the system programmer maintaining MVS should change the default TIOT size to 64K.
In addition to OS/390 restrictions on the number of ddnames that may be allocated to an address space, MSO imposes restrictions on the number of ddnames that may be allocated concurrently.
Up to 1024 ddnames may be globally allocated to the MSO address space. Global allocations are those made in the MSO JCL or allocated with the DYNAM command from the MSO Console.
Up to 256 ddnames may be allocated to each MSO user. These local allocations are made using the DYNAM command in the user's session.
To see the impact of Load Balancing on DDNAME limitations see Load Balancing.
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