Blocks

When the amount of space required is expressed in blocks, you must specify the number and average length of the blocks within the data set, as in this example:

   //  DD SPACE=(300,(5000,100)), ...
 
         300 = average block length in bytes
        5000 = primary quantity (number of blocks)
         100 = secondary quantity, to be allocated if the primary
               quantity is not enough (in blocks)

From this information, the operating system estimates and allocates the number of tracks required.

The system uses this block length value only to calculate space. This value does not have to be the same as the BLKSIZE value. If the data set is extended format, the system adds 32 to this value when calculating space.

Recommendation: For sequential and partitioned data sets, let the system calculate the block size instead of requesting space by average block length. See System-Determined Block Size.

If the average block length of the real data does not match the value coded here, the system might allocate much too little or much too much space.