An unload data set is always a variable spanned record format with sequential organization, (RECFM=VS and DSORG=PS).

The logical record length of the unload data set is intended to hold a block from the input data set plus a header, with these considerations:

1. The LRECL is calculated as being the larger of:
   1. 280 bytes, or
   2. 16 bytes + the block size + the key length of the input data set.
2. If the LRECL exceeds 32760, it is reduced to 32760.

   Note: Applications reading an unload data set should be aware that for RECFM=VS data sets, the actual length of an assembled logical record could exceed 32760 bytes, even if the LRECL was reduced to 32760 by this step.

   Note: When the input data set is a PDSE and has a block size greater than 32744, the LRECL of the unload data set is set to X, i.e. LRECL=32768. LRECL=X is used to process logical records that exceed 32760 bytes.

3. If the user supplies an LRECL larger than the one calculated here, it will be placed in the data set label; however, the size of the logical record that IEBCOPY creates will not be increased.

The block size (BLKSIZE) for an unload data set is determined by the following steps:

1. The initial block size is set to the block size supplied by the user, or if the user did not supply a block size, it is calculated as the LRECL plus 4.
2. If the block size is less than 284, it is increased to 284.
3. If the block size exceeds 32760, it is reduced to 32760.
4. The block size value is then compared with the largest block size acceptable to the output device. If the output device capacity is smaller then the block size, it is set to the maximum allowed for the output device.

Because the unload data set is unblocked, increasing the block size beyond LRECL plus 4 will not result in longer physical records or better utilization.

The block size is stored in the first control record (COPYR1) and used at load time. If the block size of the unload data set is changed after it is created, IEBCOPY might not be able to reload it.

Recommendation: Do not set the PDSU block size equal to the PDS block size or your PDSU will have very poor space utilization and performance. Let IEBCOPY pick the block size or choose a PDSU block size 20 bytes greater than the PDS block size.