How to Improve BSAM Data Transfer Rates

To optimize BSAM sequential data set transfer rates, take one or more of the following approaches:

  • If MAXSTGIO is currently defined in the initialization parameter file, review the setting and consider setting it to the 1 MB default or greater. To fine-tune and set the Number of Channel Programs (NCP) in the DCB parameter of the COPY statement, include the second positional parameter as well.
  • Increase the block size when it is advantageous to do so. Make the block size of a disk data set close to (but not more than) half-track blocking (27998 for non-extended 3390 disk data sets or 27966 for extended 3390 data sets). This improves performance by increasing the number of bytes transferred per I/O. For example, when transferring a data set with an LRECL of 80, it takes much longer to transfer 27920 bytes in 349 blocks (BLKSIZE=80) than in 1 block (BLKSIZE=27920).
Note: Exceeding half-track blocking on disk wastes a significant amount of storage capacity without improving the transfer rate.

For tape-to-tape transfers, a larger block size improves both performance and capacity. For disk-to-tape transfers, the I/O performance benefit of reblocking to an LBI block size (> 32760) may be outweighed by the CPU performance hit of transferring the data set in "record mode."