GSAM IMS symbolic checkpoint call

Among its other uses, GSAM is also useful if you need an application program that accesses z/OS® data sets to use the IMS symbolic checkpoint call.

The IMS symbolic checkpoint call makes restart easier than the z/OS basic checkpoint call. This IMS symbolic checkpoint call allows application programs to take checkpoints during processing, thereby allowing programs to restart from a checkpoint. A checkpoint call forces any GSAM buffers with inserted records to be written as short blocks. The primary advantage of taking checkpoints is that, if the system fails, the application programs can recover from a checkpoint rather than lose all your processed data. However, any application program that uses VSAM as an operating system access method and initially loads the database cannot be restarted from a checkpoint.

In general, always use DISP=OLD for GSAM data sets when restarting from a checkpoint even if you used DISP=MOD on the original execution of the job step. If you use DISP=OLD, the data set is positioned at its beginning. If you use DISP=MOD, the data set is positioned at its end.