I have been led to believe that it is possible to save a file to a save file while leaving the file active and available for use. Is this true and if so how is it done and are there any implications regarding system performance if nightly application backups were to use this method?
Yes, you can save while the file is active. If you save while active, the backup takes longer to run since it cannot allocate the object and it has to allow access to the file. So if your window to backup is short and the data files are large, you need to consider this. I have many clients that ask this question when they run a 24/7 shop. Here are some of the issues that you need to decide when you create a backup plan.
People backup data so if their system is corrupted, they can go back to a point in time and restart transactions. It is much like a physical inventory in a warehouse. A backup records what is "In-Stock". Since 99.5% of AS400 systems use relational databases, all parent child relationships need to be saved at the same time. If saving while active is used, the possibility of two dependent files having the same dependent data is reduced. Thus if you need to restore the parent file, there may be child records that no longer have relationships.
You can overcome all this by using journaling. But if you use journals, you can save the journals and the files will rebuild based upon the journals during a restore. This method works well for the 24/7 shop, but restoring takes longer since the journals need to run and update the files.
I just looked at the standard IBM save menu (Go BACKUP) and I did not see where I could enter the "save while active" request. On the SAVOBJ & SAVLIB commands, you can specify SAVACT(*YES).
I use the SAVACT(*YES) when I'm at a client site saving test data. It takes longer, but I don't interfere with the daily activity. Before you change any backup commands, take some time and develop a solid plan. IBM has a good manual on backup and recovery. Remember a solid backup tape is worth its weight in gold when you need it. A backup tape you can't restore from could be a career-changing piece of plastic.