Some Limits We Must Remember.....

MVS Limit of 2 Gigabytes for an Address Space
1.6 Gigabytes for Total Defined Virtual Pools
10,000 Open Datasets per Address Space

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Notes:

These are hard thresholds that cannot be exceeded. While HiperPools easily get us around the first two, the dataset limit has already become a problem for a few large installations. When you reach this size, the deferred (slow) close, and read only switching must be implemented. These require a solid knowledge of the applications and object usage patterns, since incorrectly using closerule ‘Y’ for many objects will generate cpu overhead for the Database Manager address space, and delay application performance/response times.

DB2 V3 and V4 will close datasets having closerule ‘N’ if DSMAX is exceeded and the datasets are not in use.