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If the LOB table space is compressed because it is defined with the COMPRESS YES option, DSN1COMP collects statistics of the individual LOB metadata from the LOB map pages and produces a report without collecting and compressing data by using the zEDC card again. 

Db2 does not compress LOBs that are less than the current page size that is used for the object. In addition, different types of files have different compression ratios. Therefore, it is useful to estimate how much your LOBs can be compressed by checking the compression ratio and data pages that are saved by using the DSN1COMP utility before compression on those LOBs is implemented. 

Db2 does not support compressing individual pieces of a linear data set. Also, because LOB table spaces are linear page sets, compression applies to the entire (non-partitioned) LOB table space. Therefore, all of the pieces (A001, A002, and so on) should be evaluated for compression savings by using DSN1COMP. However, for DSN1COMP, it is sufficient to specify only the A001 data set on the SYSUT1 DD-statement. Db2 finds the other pieces if they exist. 

A non-partitioned table space must contain all pieces when an image copy of the LOB table space is used as input to DSN1COMP. 

For a partitioned table space, the image copy can contain a single partition, a subset of partitions, or all partitions. 

A sample JCL statement that can be used to estimate space savings for a (currently uncompressed) LOB table space that uses DSN1COMP is shown in Example 3-1. Excerpts from the report that is produced also are shown. 

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