Feature request: logarithmic / exponential policy

[deliciouslytyped]

Hi, thank you for making syncoid/sanoid! I don't use them yet but I probably will.

Could / would sanoid implement a backup retention policy similar to how e.g.

• http://jekor.com/log2rotate -> http://jekor.com/log2rotate/documentation
• https://neil.fraser.name/news/2019/01/31/ -> https://neil.fraser.name/software/backup/
  work?

I also found

• https://yurichev.org/backups_log/
• https://gehrcke.de/category/technology/timegaps/ while searching for logarithmic backups though I don't know if it adds anything here.

Although I don't know how ZFSs low level storage retention stuff will interact with such a policy as opposed to a backup where individual backups are stored independently (no "diffing" of common "blocks"). - but also, maybe it doesn't matter?

I'm not sure if they all have the same algorithm but the log2rotate approach seems the most advanced (in a good sense).