Many shops rely on volume backups in order to provide their emergency and disaster recovery backups. However, there is an inherent problem with this approach.
A volume backup is a point-in-time backup that is, by definition, a physical solution. However, Endevor maintains its information and inventory logically, maintaining independence from the actual physical location of the actual elements. Therefore, unless all the information for an application happens to be stored on the same physical DASD device when the volume backup is occurring, there is a distinct possibility of serious synchronization problems.
To illustrate, my base libraries may be on SYSVL1, the delta libraries on SYSVL2, and my MCF libraries on VSMVL1. The volume backups for these particular devices occurred at 17:23, 18:34, and 19:02 respectively.
During that same time, I also have a regularly scheduled Endevor batch job that runs every hour on the hour to check for approved packages that meet the time windows for execution. The jobs ran, therefore, at 17:00, 18:00 and 19:00.
During that time, it found a job to execute at 18:00 and moved 200 elements from one stage to the next. At 19:00, it found another job that executed GENERATEs against 100 elements.
Based on this information, what do my volume backups contain for my system? If I restored these datasets from the backups, my base library would not match my delta library. And neither of them would match my MCF. In other words, I would have a serious problem to address in the middle of what is already a disaster (i.e. the event that forced the restore in the first place).
Using full and incremental UNLOADs solve this problem by addressing entire applications at the time of the unload separate from the physical location. Although time-consuming, the solution provided in the event of a disaster is comprehensive, complete, and correct. If my UNLOAD took place at 17:23, then no changes can occur to my application, regardless of file, until the UNLOAD is complete. This ensures that, in the event I need the data to RELOAD, I have complete information for my base, delta, and MCF files.
Am I advocating no longer doing volume backups? No, not at all. I recommend continuing to perform volume backups but keep the UNLOAD files as backup for your backup! During a disaster recovery exercise, the steps I recommend are to restore the volume backups and then run an Endevor VALIDATE job. If the job comes back clean, you’re good to go! But if there’s a problem, you have the necessary files to restore things to a stable state.
A final note. The format of the full and incremental UNLOAD files is the same as that created for the ARCHIVE action. Therefore, a secondary use for the UNLOAD file can be to restore “accidentally” deleted elements; in other words, even though not designed to restore specific elements, the fact is they can be restored by using the RESTORE SCL the same as you would against an ARCHIVE file. Just use the UNLOAD file instead!