The work file is dynamically allocated during processing and is used to temporarily save index information prior to building table index pages.
The default number of volumes available for allocation of the work file is one that is normally sufficient but with very large tables the space required could exceed the total space of a single volume. To reduce the primary allocation, in this case it is necessary to manipulate the number of records or rows.
When secondary indexes are to be built, the block size of the work file is critical: the elapsed time (with similar system activity) of a job using an optimum block size can be up to half that of the same job using the default block size of 4096 bytes.