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SDB is an IBM facility which chooses the optimum BLKSIZE for a physical sequential data set based on its device type when the user codes BLKSIZE=0 in JCL.
Because a SUBSYS data set looks to the system like a SYSIN/SYSOUT data set, SDB computes the BLKSIZE by adding four to the LRECL instead of choosing the BLKSIZE appropriate to the actual device. This leads to very poor blocking factors and defeats the purpose of compression.
Use PST, which works correctly, or code the actual BLKSIZE on the SUBSYS DD statement and its associated ddname.
If you code BLKSIZE=0 on the SUBSYS DD statement and the associated ddname, the job abends because the BLKSIZE is incompatibly calculated on the two DD statements.
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