It's a bit tricky to come up with simple description. IMHO examples are
the best way how to demonstrate the functionality, so, please find below
what I would include in the documentation/help for the Simple Backup
Suite.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Purging
Is the instrument to keep your backup lean and prevents your data storage from
overfilling in conjunction with your overall backup strategy. It's triggered at
the end of a successful backup execution. The removal of old backups can be
configured in two ways:
- Simple cut-off: Erase all backups older than "n" days.
To prevent unwanted data lost in the case when using INCREMENTAL backups,
Sbackup will only remove all the backups that are before the parent (FULL) of
the INCREMENTAL (child) backups. In other words,
the result of modulo operation of the "n" days and the frequency of FULL
backup should be 0 to achieve desired effect.
Example 1 (incorrect setting):
Do FULL backup once every 7 days ("f"=7)
Simple cut-off: Erase backups older than 10 days ("n"=10)
n mod f = 3 ... which is NOT 0.
In this case, only backups older then 14 days shall be purged (14 mod
7 = 0), because if it would done that simple cut-off, it would leave 2
INCREMENTAL (orphans) backups without their FULL (parent) backup
2018-05-01.Ubuntu.ful/ <------ CUTOFF
2018-05-02.Ubuntu.inc/ <------ CUTOFF
2018-05-03.Ubuntu.inc/ <------ CUTOFF
2018-05-04.Ubuntu.inc/ <------ CUTOFF
2018-05-05.Ubuntu.inc/ <------ CUTOFF
2018-05-06.Ubuntu.inc/
2018-05-07.Ubuntu.inc/
2018-05-08.Ubuntu.ful/
2018-05-09.Ubuntu.inc/
2018-05-10.Ubuntu.inc/
2018-05-11.Ubuntu.inc/
2018-05-12.Ubuntu.inc/
2018-05-13.Ubuntu.inc/
2018-05-14.Ubuntu.inc/
2018-05-15.Ubuntu.ful/
Example 2 (correct setup):
Do FULL backup once every 7 days ("f"=7)
Simple cut-off: Erase backups older than 7 days ("n"=7)
n mod f = 0
which means, in this case, it will keep backups up to 2 weeks. Once
the 3rd FULL backup will be completed, it shall purge the backups from the 1st
to 7th of May.
- Logarithmic
....as existing...
~~~~~~~~~~~~~~~~~~~~~~~~~~~
as not sure again what it would be the behavior.
Perhaps in the case of using INCREMENTAL backups, it should keep
nearest FULL backup.
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https://bugs.launchpad.net/bugs/1770528
Title:
Purging of old backup - Simple cut-off is not happening
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