Updating of the Datacom CXX value fpr DATAHU (High Used Track)

Products

• Datacom
• Datacom/AD
• Datacom/DB

Issue/Introduction

The DBUTLTY function DATASCAN OPTION=SET_DATAHU will initially set a high-water mark value in a data area. As the volume of data grows in that data area, will the DATAHU value automatically change, or do we have to periodically run the DATASCAN to reset the current DATAHU value?

Environment

Datacom/DB

Datacom/AD

z/OS

Resolution

Processing of the DATAHU value is meant to be completely internal, and therefore there is nothing that a user needs to do once the setting has been enabled and the DATAHU tracks value is set the first time.

Consequently, as data is added to the file, the DATAHU value increases as needed, having values of groups of 64 tracks. Note that the value will not automatically decrease, so if you have a process that adds records to tracks 1 to 99, the DATAHU value will be 128 (the next 64-track boundary above the actual 99 tracks used). If you then delete data from tracks 2 through 96, the high-used value will still be 128, and if you then delete data from tracks 97 through 99, so that there is only live data in track 1, the high-used value remains at 128, since that reflects the highest track used (the actual high track was 99, so the highest DATAHU value is 128).

The short answer to your question, then, is that you do not have to do anything special to maintain the DATAHU value as the file usage grows. You can always run the DATASCAN if the file shrinks, or you can use Backup/Load to do the same thing, with the added benefit of compressing and defragmenting the data if you have a lot of empty space. However, this may only be useful if you have a large data area and have removed many of the records within it. This DATAHU value is not very useful to any user, as its management and usage are all internal.

Now, if you LOAD the data area from a backup or extract (with or without using INIT), DBUTLTY will reset the DATAHU value to the nearest 64-track value above the actual usage count. For example, if you loaded 99 tracks of data, the CXX would show (under the data area column)

HIGH USED TRACK -        128

Then, if all those records were deleted from tracks 2-99, and you next ran a BACKUP and a LOAD, the CXX would show

HIGH USED TRACK -         64

As you can see, there is no need to run another DATASCAN, as the value is maintained automatically.

Additional Information

For more information about the DATAHU (High-Used Track) value, please see the following documentation topics:

For information on improving sequential performance with GSETP/GETPS commands, please see the section Optimizing Sequential Processing in the Optimizing Datacom/DB Performance topic.

For details on how to enable the DATAHU feature for use in your MUF, please see the topic CXXMAINT ALTER OPTION2= (Activating Other Features).

For details on resetting the DATAHU value once the feature has been activated, please see the general topic Using DATASCAN to Activate DATAHU Online and the DBUTLTY function DATASCAN OPTION=SET_DATAHU.

As always, please contact Broadcom support for Datacom if you have further questions.