Clarify backup wording in context of potentially running out of space

[nomandera]

For context I just ran out of disk space. I quickly cleaned this up but I reviewed my backup settings and as it stands I dont think GUI wording is such that users can fully grasp what it means.

When I ran out of space I had local backups only set to : Retention Days: 3 which resulted in two FULL backups and a series of incremental backups in between. Sorry i dont have exact dates as I panic fixed the issue but it was from memory 15 days elapsed between the full backups.

The web GUI says

Retention Days: This is the minimum time backup data is kept for. The box makes an incremental backup most nights, which requires that previous backups back to the most recent full backup be preserved, so backup data is often kept much longer than this setting. Full backups are made periodically when the incremental backup data size exceeds a limit.

It do not think the wording Full backups are made periodically when the incremental backup data size exceeds a limit. naturally fits with an intuitive understanding of ending up with two full backups, 2 weeks of incremental backups and a second full backup that fills the box up to breaking point.

Can we clarify what the incremental backup data size exceeds a limit precisely is and then potentially I can have a stab at some improved wording once I understand the process better.

[ddavness]

The retention period actually only applies to old backups.

• Your first backup will always be a full backup (obviously)
• Then your next backups will be incremental. When the sum of the sizes of the incremental backups exceeds half of the size of the corresponding full backup, we create a new full backup instead. https://github.com/mail-in-a-box/mailinabox/blob/8664afa99798c9dbd7b52cf67da7e90b3280bbf0/management/backup.py#L159-L162
• Only then the retention period kicks in. Since now we have a new full backup, the old full backup (and respective incremental backups) are no longer needed - and as such, for convenience, the retention period begins the countdown. Once past that retention period, the "old" backup and respective incremental backups are then deleted.