Multiple copies of an area data set

When multiple copies of an area data set are used, the ADS is referred to as a multiple area data set (MADS).

Each copy of a MADS contains exactly the same user data. Fast Path maintains data integrity by keeping identical data in the copies during application processing. When an application program updates data in an area, Fast Path updates that data in each copy of the MADS.

When an application program reads data from an area, IMS always attempts to read from the first ADS shown in the RECON list. If the first ADS is not available or if it is in a long busy state, IMS attempts to read from each subsequent ADS in the list until an available ADS is found. If all of the ADSs are in a long busy state, IMS uses the first ADS in the list.

All copies of a MADS must have the same definition but can reside on different devices and on different device types.

If your MADS copies reside on different devices, place the first ADS registered in the RECON data set on your fastest DASD for the best read performance. Subsequent copies of the ADS can reside on slower DASD without affecting overall read performance.

Using MADS can also be helpful in DASD migration; for example, from a 3380 device to a 3390 device.

To create a copy of an area data set, issue the DBRC command INIT.ADS. In the AREA(name) parameter, specify the name of the original ADS as it is recorded in the RECON data set. Issue the INIT.ADS command for each additional copy. For more information about creating MADS, see the INIT.ADS command in IMS Version 15.4 Commands, Volume 3: IMS Component and z/OS® Commands.

If an ADS fails to open during normal open processing of a DEDB, none of the copies of the ADS can be allocated, and the area is stopped. However, when open failure occurs during emergency restart, only the failed ADS is deallocated and stopped. The other copies of the ADS remain available for use.