What is the difference between SCHEDULED and non-SCHEDULED compression?
SCHEDULED compression specifies that compression is to take place either when the dataset is next loaded or opened for output. Because the dataset may contain both compressed and uncompressed records, and at any rate must be assumed to have been defined as an uncompressed dataset, no change to key position or record size is possible. Accordingly, there can be no compression before the last key, and should a record grow on compression, the smaller uncompressed record must be written instead, so for FDT compression, there must be an ICB. Non-SCHEDULED compression , also known as IMMEDIATE compression, means that the entire dataset is compressed when compression is implemented. The DEFINE may change, notably by moving all the keys to the front of the record, in order to support compression of data before the keys. SCHEDULED compression applies only to VSAM datasets. Somewhat similar control of the compression process is achieved by the Physical Sequential Transparency (PST) by means of a
