Purge Inter-month Incremental Backups

New retention policy deletes old backups in a smart way announced this, but seemingly the manual hasn’t caught up. Here’s a screenshot example of something that sounds an awful lot like what you’re asking for:

1D:U,1W:1D,4W:1W,12M:1M is retention rules that progressively thin out backups depending on their age.

  • For 1 day, keep all
  • For 1 week, keep 1 per day
  • For 4 weeks, keep 1 per week
  • For 12 months, keep 1 per month
  • After 12 months, keep no backup

Your preferences differ, but your example near the top of the post is roughly 1W:1D,4W:1W,12M:1M,U:1Y

You know about the transient file issue. One additional caveat here is there’s no calendar. It’s all intervals.

C:\Program Files\Duplicati 2>Duplicati.CommandLine.exe help retention-policy
  --retention-policy (String): Reduce number of versions by deleting old
    intermediate backups
    Use this option to reduce the number of versions that are kept with
    increasing version age by deleting most of the old backups. The expected
    format is a comma separated list of colon separated time frame and
    interval pairs. For example the value "7D:0s,3M:1D,10Y:2M" means "For 7
    day keep all backups, for 3 months keep one backup every day, for 10
    years one backup every 2nd month and delete every backup older than
    this.". This option also supports using the specifier "U" to indicate an
    unlimited time interval.

C:\Program Files\Duplicati 2>