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Monday night's backup is 20% faster

We have about 20 AS/400 (models 7xx and 8xx) installed here in Switzerland within CIBA, and we are backing up data on a daily(MO-TH), weekly (FR) and monthly basis using the "GO BACKUP" OS/400 tasks.

We are also backing up data paths. After each backup we IPL the system. We've found out this strange thing years ago but we are not able to explain why. Here's the description: The first backup of the week (after the week-end) running on Monday night, always runs 20% time faster and we do not know why. The same thing appears on all 20 systems. Can you help me understanding what is happening?

The longest process in the backup model is the building of the list of objects to be backed up. If the system has a full save on Friday, and Monday is the first backup after that, the list is built from scratch. On Tuesday through Thursday, the list is compared and updated based on the previous list, thus the increase in time for that range of backups. If you had the time to watch your backup, you could work with the job and periodically watch the program stack. You would probably see the increase in time in a few specific programs. It has been about 7 or 8 years since I did this, and it was on a CISC machine, so I'm sure that what I used to see is no longer the same names, but the processes are the same.

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