Summary of PDSE Storage Requirements

When planning for PDSE processing, consider the following storage requirements:

The storage necessary for directory information is obtained from storage that is generally available to the data set. Because the directory expands dynamically, storage for the directory is obtained whenever needed. The directory storage need not be limited to the primary extent, but can be obtained from any available storage.
For each PDSE member, a variety of information is retained in the directory (such as attributes, statistics, and status). The directory storage required to support any single member is variable, as is the storage required to support alias names. For a medium-sized PDSE containing approximately 150 members, approximately 12 pages (4096 bytes per page) of directory storage is required.
Deleting of a PDSE member can, in some rare cases, actually increase the amount of directory space used by the remaining members. This can cause the total amount of directory space required to be larger after the delete.
The only time this problem could occur is if nearly all the storage allocated for the PDSE is used. If a STOW delete or rename of a member requires that the PDSE be extended and the extend fails, STOW will return an error return code. This return code will indicate why additional DASD storage could not be allocated.
When allocating a PDSE where generations are supported, allocate additional space to store the generations.