Purging JAMS history is normally completely automated. When JAMS is initially installed, a job named JAMS_PURGE is created which is scheduled to run every Sunday.  This job purges the JAMS History and then does a CONVERT/RECLAIM on the HIST.DAT file to reclaim space from the deleted records.

Verify that the JAMS_PURGE job is successfully running.

If JAMS_PURGE is running, it may be that a user is keeping more history than desired.  Users can adjust the default values for the parameters to JAMS_PURGE to change the amount of history a user keeps.

It may also be that the HIST.DAT file grew so large because of an unusual event and the file is mostly free space. The JAMS_PURGE job won't shrink the physical size of the file. To shrink the file while preserving the history, perform the following:

$ MCR JAMS_EXE:JAMS_MASTER STOP SCHEDULE/NORESTART
$ SET DEFAULT JAMS_DATA:
$ ANALYZE/RMS/FDL/OUTPUT=HISTOPT.FDL HIST.DAT
$ EDIT/FDL/NOINTERACTIVE/ANALYSIS=HISTOPT.FDL HISTOPT.FDL
$ CONVERT/FDL=HISTOPT.FDL/STATS/NOSORT HIST.DAT HIST.DAT
$ PURGE HIST.DAT
$ MCR JAMS_EXE:JAMS_MASTER START SCHEDULE 

Note: This may take a long time. If you don't need to retain the history and simply need to shrink it, the following can be utilized: 

$ MCR JAMS_EXE:JAMS_MASTER STOP SCHEDULE/NORESTART
$ SET DEFAULT JAMS_DATA:
$ DELETE HIST.DAT;*
$ CREATE/FDL=HIST.FDL HIST.DAT
$ MCR JAMS_EXE:JAMS_MASTER START SCHEDULE