Data sets with fixed or variable records can be reblocked to a different block size by previously allocating the desired block size on the receiving volume. No reblocking can be performed when loading or unloading. Also, no reblocking can be performed on data sets with variable spanned or variable blocked spanned records.

When moving or copying data sets with undefined record format and reblocking to a smaller block size (that is, transferring records to a device with a track capacity smaller than the track capacity of the original device), you must make the block size for the receiving volume equal to or larger than the size of the largest record in the data set being moved or copied.

When copying data sets with undefined record format to a device with a larger track capacity, IEHMOVE will not reblock the output data set to a larger block size. IEHMOVE simply copies the source data set to the target data set.

However, if the target data set is preallocated with a larger block size than the source data set, the data set becomes unusable because the source block size is used during the copy.

Blocked format data sets that do not contain user data TTRNs or keys can be reblocked or unblocked by including the proper keyword subparameters in the DCB operand of the DD statement used to previously allocate space for the data set. The new blocking factor must be a multiple of the logical record length originally assigned to the data set.

Related reading: For information about user data TTRNs, see z/OS DFSMS Using Data Sets.