This topic provides guidelines that you can use when you submit multiple jobs that update a particular GDG:

• No two jobs running concurrently can refer to the same GDG.
• For batch or dynamic allocation jobs that specify relative generation numbers, the system enqueues the GDG base name as shared or exclusive, depending on the highest disposition that is used in the job. The GDG base name is exclusive if the highest job disposition is NEW or MOD. The GDG base name is shared if the highest job disposition is SHR. This safeguard prevents concurrent users from updating the GDG by adding or deleting generation data sets while other users are using the GDG.
• For batch or dynamic allocation jobs that use absolute generation data set names, the system does not enqueue the GDG base. Multiple users are able to update the GDG by deleting or adding generation data sets at the same time. This situation does not affect the integrity of the GDG or generation data sets. However, jobs that use relative generation numbers might obtain the wrong generation, because the numbers can change. Even if you use absolute generation numbers, a job might accidentally replace a generation data set that another job is using.

  The only time that you can use absolute generation numbers is when you need to run concurrent jobs that use the same GDG and at least one of the jobs uses a disposition of NEW or MOD. Ensure that the jobs do not accidentally overlay a generation data set that another job is using.

Restriction: Be careful when you update GDGs because two or more jobs can compete for the same resource and accidentally replace the generation data set with the wrong version in the GDG. To prevent two users from allocating the same absolute generation data set, take one of the following actions:

• Specify DISP=OLD.
• Specify DISP=SHR and open the data set for output.