Average record length

When the amount of space required is expressed in average record length, you must specify the number of records within the data set and their average length. Use the AVGREC keyword to modify the scale of your request. When AVGREC is specified, the first subparameter of SPACE becomes the average record length. The system applies the scale value to the primary and secondary quantities specified for the SPACE keyword. Possible values for the AVGREC keyword follow:
  • For U, use a scale of 1.
  • For K, use a scale of 1024.
  • For M, use a scale of 1048576.
When the AVGREC keyword is specified, the values specified for primary and secondary quantities in the SPACE keyword are multiplied by the scale and those new values will be used in the space allocation. For example, the following request results in the primary and secondary quantities being multiplied by 1024: 
   //  DD SPACE=(80,(20,2)),AVGREC=K, ...
 
                         80  = average record length in bytes
    80 * 20 * 1024 = 1.6 MB  = primary space
    80 *  2 * 1024 = 160 KB  = secondary space, to be allocated if the
                               primary space is not enough

From this information, the operating system estimates and allocates the number of tracks required using one of the following block lengths, in the order indicated:

  1. 4096, if the data set is a PDSE.
  2. The BLKSIZE parameter on the DD statement or the BLKSIZE subparameter of the DCB parameter on the DD.
  3. The system determined block size, if available.
  4. A default value of 4096.
Start of changeFor an extended-format data set, the operating system uses a value 32 bytes larger than the preceding block size. The primary and secondary space are divided by the block length to determine the number of blocks requested. The operating system determines how many blocks of the block length can be written on one track of the device. The primary and secondary space in blocks is then divided by the number of blocks per track to obtain a track value, as shown in the following examples. These examples assume a block length of 27998. Two blocks of block length 27998 can be written on a 3390 device: 
   (1.6MB / 27998) / 2 = 30 = primary space in tracks
   (160KB /  27998) / 2 =  3 = secondary space in tracks
Note that for extended format, you must allow for an additional 32 bytes per block. Int that case, the maximum block size that you can use with two blocks per track would be 27966.End of change

In the preceding calculations, the system does not consider if it is a compressed format data set. This means the calculation is done with the user-perceived uncompressed block size and not the actual block size that the system calculates.