On a Selective Copy, if you have selected Automatically select Full Backups at frequency option with Advanced - Every <x> Cycle(s) option, then the job selection will be done based on the Mod Logic.
The Mod Logic is implemented as follows:
N mod x
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If N mod x is 1 then only first full backup jobs will be selected.
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If N mod x is 0 then the last full backup jobs will be selected.
Where:
N - is the cycle number of a backup job as seen under Cycles/Sequence column in the Job for Storage Policy/Storage Policy copy window, which will appear by right-clicking the Storage Policy, pointing to All Tasks and then clicking View Jobs.
x - is the number of cycles defined in the Every <x> Cycle(s) option in the Advanced Options dialog box.
Example:
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If N = 3 and x = 2, then N mod x => 3 mod 2 = 1 - the backup will qualify as a first full backup.
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If N = 2 and x = 2, then N mod x => 2 mod 2 = 0 - which means that backup will qualify as last full backup for the copy, if last full backup option is selected.
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If you have specified Every <x> Cycles (s) values as 4 (i.e., x = 4) during selective copy creation and selected First Full Backup option and during copy creation if the number of backup cycles on a source copy is 16 (i.e., N = 16), then:
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16 mod 4 = 0 and this backup will not qualify as first full backup for this newly created selective copy as the mod value is 0.
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If the backup cycle value is 17 (i.e., N = 17), then 17 mod 4 = 1, this backup will be qualified for copy as first full backup for this copy.
In case of backup cycles N = 18, 19, 20 e.t.c., these backups will not qualify for copy selection because of mod logic.
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