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In Full backup retention, a specified number, or age, of full backups are retained. When full backups are purged, the entire backup set is also deleted. F = Full
D = Differential
I = Incremental
M T W T F M T W T F M T W T F M T W T F M T W T F M T W
F I I I I D I I I I D I I I I D I I I I D I I I I F I I 
e.g, Retain 1 Full backup.
After retention, the entire previous backup set is deleted M T W T F M T W T F M T W T F M T W T F M T W T F M T W
F I I I I D I I I I D I I I I D I I I I D I I I I F I I
Differential retention
Specify a number of differential backups to retain by age or number, M T W T F M T W T F M T W T F M T W T F M T W T F M T W
F I I I I D I I I I D I I I I D I I I I D I I I I F I I
For example, Retain 2 differential backups.
After retention, the most recent 2 differential backups are retained. Incremental backups that are linked to previous differential backups are deleted along with the deleted differentials.
M T W T F M T W T F M T W T F M T W T F M T W T F M T W
F I I I I D I I I I D I I I I D I I I I D I I I I F I I
Incremental retention
Specify the number of incremental backups to retain. If by deleting an incremental the backup chain is broken this causes consolidation of the oldest retained Incremental.
M T W T F M T W T F M T W T F M T W T F M T W T F M T W F I I I I D I I I I D I I I I D I I I I D I I I I F I I
For example, Retain 4 incremental backups.
After retention, the most recent 4 incremental backups are retained. The oldest retained incremental is not be valid on its own as it requires the previous 2 incremental backups to complete the chain. To ensure backup integrity the 2 older incremental backups are consolidated with it to create a new incremental backup.
M T W T F M T W T F M T W T F M T W T F M T W T F M T W
F I I I I D I I I I D I I I I D I I I I D I I I I F I I
--->I I F I I
Synthetic Full / Incrementals forever
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For example, Incrementals forever with retain 4 incrementals. For the first week, M T W T F
F I I I I
The next Monday, to retain 4 Incrementals, the Full is consolidated with the first incremental to create a new Synthetic Full. M T W T F M
F I I I I I
->F I I I I
On Tuesday the consolidation step is repeated again on the next incremental image. M T W T F M T
F I I I I I
-->F I I I I
This process is repeated forever. After the first Full only Incremental backups are required.
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Backup Type | Schedule | Retention |
Monthly Full Backup | Run on the 1st Monday of every month | 26 Weeks |
Daily Diff Backup | Run on Mon, Tues, Wed, Thu, Fri (except 1st Monday) | 30 Days |
Incremental Backup Set
A Full backup is created periodically followed by daily Incremental backups.
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Backup Type | Schedule | Retention |
Initial Full Backup | Initially created when the first Incremental is scheduled to be created | A Synthetic Full is created after the first 30 Incrementals |
Daily Inc Backup | Run Every Mon, Tues, Wed, Thu Fri | 30 |
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