Hi,

I am trying to calculate the space parameter for a KSDS that will be using compression.

In the DFSMS Using Datasets Manual, it states that the CI for a compressed file with non-spanned records will contain 3-byte field that will contain the compression information.

Are the CIDF and RDF control information fields also present?

Any additional assistance on how to calculate space for a VSAM dataset would be greatly appreciated.

Thank you.

Hello Girish,

There is no formula I know of for calculating space on a VSAM compressed data set.  A compression token will be updated to the catalog after enough information is written to the data set to generate a token.  If the initial data loaded to the data set is representative of the data in the data set, a good token will be derived but that is about all we can say.  The book is trying to warn you of the overhead of compressing a record (and yes, they are compressed a the record level and not at the CI level) and each record will have that overhead.  So, of course, with smaller records this overhead would be more noticeable than on larger records. The VSAM Demystified has a more detailed description of compression and how it works and some warnings.  Of course, after you load the data set is is easy to see how well it compressed with a LISTCAT that shows the user data size vs. the compressed user data size.
My apologies for not being able to answer your specific question.
Helen Witter
VSAM Development