Share options of (2,3) allow one address space to update the dataset and any number to read it. However, it is the user's responsibility to provide read integrity. Reading without integrity can cause problems with stale data and incomplete pointers.
Share options of (3,4) allow a dataset to be read and written by any number of users. However, it is up to the application to provide read and write integrity using ENQ or RESERVCE macros to serialize access. Not only is this complicated, it's asking for trouble.
You may also be opening yourself up for performance problems if the dataset is heavily used. One "hot" system could keep other LPARs from getting to the file. However, with the new architecture of storage devices and the development of faster channels and parallel access volumes (PAVs), the days of moving datasets off of heavily used volumes are fading fast.
If you intend to share the dataset for read and write you will have to use the serialization techniques mentioned above. This could turn into a performance problem because enqueuing across LPARs is not cheap. You may also get into a situation where a program hangs up while holding the enqueue, thus shutting all the other readers and writers out.
MORE ON VSAM:
VSAM basics (Oct. 31, 2006)
Virtual Storage Access Method (VSAM) still holds mountains of the world's data sets. This column explains how VSAM works.
CICS-VSAM tuning options
Expert Robert Crawford discusses a user's decreased system performance in relation to its VSAM stats.
This was first published in October 2007