Mirfster
I think some of the "mumbo jumbo"
ie the part that says:
"The number of increments in a set is only equal to the "Daily backups in each set" if a multiple of 7. For instance choose 27, and you only get 21 increments, whereas 28 gives 28 increments. Or choose 3, and it gives 7 increments. (This assumes that "Full backup is allowed on" is not set to everyday)",
is really relating to unusual combination of settings where the number of incrementals does not match or equate to the full backup period eg 7 incrementals matches a full backup every 7 days, so don't be confused by that.
Use settings that are relative to each other eg weekly (7 day) full backup with 7 incrementals.
Hopefully the following simplifies it, keeping in mind the choice of full backup on a specified day or every day will also interact with these settings.
If you wanted to do a full backup every day, & only retain 1 historical backup, then select
Number of daily backups contained in each set is 1.
Number of rotating backup sets is 1.
If you wanted to do a full backup once every 7 days, then daily incremental backups every other day, & only retain 1 historical backup, select
Number of daily backups contained in each set is 7.
Number of rotating backup sets is 1.
If you wanted to do a full backup once every 14 days, then daily incremental backups every other day, & only retain 1 historical backup, select
Number of daily backups contained in each set is 14.
Number of rotating backup sets is 1.
If you wanted to do a full backup once every 28 days, then daily incremental backups every other day, & only retain 1 historical backup, select
Number of daily backups contained in each set is 28.
Number of rotating backup sets is 1.
For any of the above, if you want to retain more historical backup sets, then change the
Number of rotating backup sets to 2 or 3 or 4 as desired
So let's say the full backup is set to run on Friday only, & the backup time is set for 11pm.
On the first Friday night at 11pm a full backup is done, this may take many hours or even time out if there is a lot of data to backup.
On the first Saturday night at 11pm if the full backup did not complete the night before (timed out as per time out setting or reached 24 hours run time), then the full backup is continued on from where it left off.
If the full backup did complete, then an incremental backup is done, so only the additions & changes of data are added to the first full backup.
These are added as incremental backups, in a seperate incremental backup file with a date identifier.
Then every day of the backup period (being the Number of daily backups contained in each set), ie 7, 14 or 28 days, an incremental backup is done.
Then on the Friday (being either day 8, 15 or 29 in examples above) at 11pm a new full backup is done.
After that incremental backups are done each day, & so the cycle continues on.
Note, when a full backup is done:
If Number of rotating backup sets is set to 1, then the original (first) backup is deleted.
If Number of rotating backup sets is set to 2, then the original (first) backup set is retained & a new backup set is created/started.
If Number of rotating backup sets is set to 3, then the first & second backup sets are retained & a new backup set is created/started.
If Number of rotating backup sets is set to 4, then the first, second & third backup sets are retained & a new backup set is created/started.
.... & so on.
Note that if a full restore is needed, the full & incremental backups are used, so all data is restored.
A "practical" backup approach may be to configure for a Full backup every 7 days & daily incremental backups in between, with 2 rotating sets, so a new full backup is created each Friday & you retain a complete 7 days worth of backups for historical purposes.
You could even extend that to retaining 4 sets of historical backups, in case you wanted to restore some individual files from 25 days ago.
Another approach is to have 2 backup USB disks, & swap the disk every Friday (during the day) so the new full backup (on Friday night) is done to the alternate USB drive, and that alternate drive is used for the rest of the 7 days period for daily incremental backups.
How often you want to create a full backup & how many daily incrementals you want to have in a set & how many historical sets & whether you swap disks off site etc, is really a part of your overall backup protocol or strategy for the site/server in question.
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