restic 0.19.1 compiled with go1.26.5 on darwin/arm64
For simplicity, assume one backup a day, followed by a run of restic forget --max-unused [ value ] --prune. In order to satisfy the --max-unused parameter, restic must download some data from the repository. I understand why that needs to happen.
My question is this: suppose I use --max-unused unlimited 6 days a week, but on the 7th day, I specify --max-unused 0. Does this make any difference in how much data must be downloaded over several weeks? Instead of using --max-unused 0 every day?
Reason for question: I use cloud storage for the repo; I would like to optimize for minimum storage and minimum download bandwidth.
over several weeks the total amount prune has to read is basically the same. unused space still accumulates, day-7 with --max-unused 0 just pays for all of it in one hit.
so you dont really save download bandwidth by spreading unlimited/0 like that. you mostly change when the spike happens (one fat prune vs smaller daily ones).
if you care about cloud egress, something like --max-unused 5% (or 10%) every day is usually less painful than weekly 0. unlimited all week then 0 is the spikiest option.
storage side: unused packs sit around until that strict prune, so average bytes stored is a bit higher with the 6+1 pattern too.
There are two cases when snapshots are removed (by forget):
Completely unused pack files → Those are always removed by the next prune run (which doesn’t need to read/download anything)
Partly used pack files, i.e. packs where blobs which are used and blobs which are no longer used → These packs can be repacked to remove the unused blobs. And for this, the --max-unused option can reduce repacking.
It is true that --max-unused 0 will repack all party used pack files. The advise is to set it to some percentage of the total repo size, e.g. 5%.
About the original question:
It can be that a weekly --max-unused 0 does need to download all partly used packs and hence this is just shifts the point in time when the repacking happens compared to a daily --max-unused 0.
However, it can also be that a daily --max-unused unlimted (or some high enough value) will not repack a partial used pack, but during the week (and more removed snapshots), this partial used pack turns into a unused pack and will be removed without the need to read it at all.
So, as with all complex situations, the answer is: It depends
