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How many backups of my backups do I have? (lemmy.dbzer0.com)

submitted 4 months ago* (last edited 4 months ago) by dataprolet@lemmy.dbzer0.com to c/math@lemmy.ml

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So this is bugging me for a while and I'm just do dumb to get how I solve this, but here's the situation:
Given I take a local backup of my system daily and have a retention policy that keeps a backup of the past 7 days each, a backup of the past 4 weeks each and a backup of the past 6 month each. That's either 17 backups or less if you consider some backups being counted as a daily and weekly or as a weekly and monthly. But that's not that important.
The interesting part is, that I also take a remote backup of my local backup daily, which has the same retention policy, so it's cascading. Here there is obviously a huge overlap of backups, but I can't wrap my head around, how I calculate this.
Is anybody willing and/or interested to solve this for and with me?

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[–] davel@lemmy.ml 3 points 4 months ago* (last edited 4 months ago) (1 children)

An important question that falls outside of math is: which of these are incremental backups, and which are full backups? Because the two shouldn’t be summed as of they’re comparable: they’re apples & oranges. Also, which of these backups share the same storage medium, because if half of these are all on the same disk and that disk dies, you’ve lost half your backups thanks to one failure.

[–] dataprolet@lemmy.dbzer0.com 0 points 4 months ago

All backups are incremental, so all local backups together "count" as one full backup and all remote backups "count" as one full backup. And I take a backup of my SSD to a local HDD and from that to a hosted storage solution. I also have an old full backup on an external disk which is in my book shelf. I guess I'm pretty much fulfilling the 3:2:1 rule, right? By the way I literally counted all backups and there are 51 unique (incremental) backups,