Database processing

Access methods

The following access methods are available for reading the input data set group or area.

The start I/O access method is the default access method.

• start I/O access method (default method)

  The start I/O access method reads OSAM, ESDS, and KSDS data sets at speeds approaching the physical capabilities of the input device, with a significant reduction in CPU time.

• native access method (OSAM/VSAM)

  The native access method reads the data set at the same speed as an optimally tuned IMS Database Image Copy utility, but you can still take advantage of other product features (such as asynchronous processing and data compression). If you select this method, you can also control the input data set buffer pool size. For more information, see Buffers.

  When processing a VSAM KSDS, a HISAM overflow data set, or an overflow data set for a secondary index with duplicate keys, the utility uses control interval access to improve performance.

• EX CP-level access method

  The EXCP-level access method is available for OSAM databases. This access method reads the database at speeds that approach the physical capabilities of the input device. If you select this access method, the utility allocates input buffers above the 16 MB line.

  Important

  The BMC Database Utilities Subsystem (DBUSS) is no longer used for fast I/O. However, the DBUSS can be used to enable certain authorized services when STEPLIB is not APF authorized and to allow a compressed image copy data set to be read by a non-BMC utility, as explained in Using-the-Subsystem-Data-Set-Interface-ICPSSDSI.

  The start I/O access method achieves the best performance and should be used in almost all cases. The only circumstance that might affect the use of the start I/O access method is if many small database image copies are competing for resources that could be used for other large database image copies that are executing. In this circumstance, you can balance the image copy load by specifying the MINCYL keyword for the smaller database image copies. This specification forces the smaller database to use the EXCP access method, freeing some resources for the large database image copies.

  Important

  With small databases the start I/O access method may not noticeably affect run time but may consume considerable system resources. You may find it best to use the start I/O access method and large buffer sizes for large databases and use the native access method and one track of buffers for small databases.

Buffers

When you are using the native access method, you can specify the number of buffers for an input data set group or area manually.

The utility always allocates buffers below the 16 MB line when you are using native access methods. The number of buffers that you specify affects the performance of the utility. Generally, the more buffers you use, the better the performance. However, if you have a heavy paging environment, fewer buffers may give you better performance.

When EXCP is used to read an input file, the number of buffers is always equal to two cylinders of input data. The utility allocates these buffers above the 16 MB line.

When the start I/O access method is used to read the input file, the number of buffers is always equal to two cylinders of input data. These buffers are allocated in PAGE FIXED storage. The utility allocates these buffers above the 16 MB line.