My guess would be that this are mostly some buffer which are preallocated. Although that is hard to tell from the numbers.
That looks very odd. We definitely don’t read files twice. The part of restic which reads a file, has essentially no idea whether a repository get’s compressed or not.
What is also rather strange is that borg reads more data in the create-2 test case when backing up to the high latency backend compare to the normal latency one. I don’t think that should happen?
According to your test scripts repository, the first data set is 9.3 GB so why is every backup tool reading twice that amount? And for the text data set (10.2GB?) we’re now at 2-4x as much data. Neither scenario makes sense.
I also don’t see any flushing of the filesystem cache between the test runs (at least between different backup tools that would be a good idea).
Yes, currently new pack files are created as temporary files to not have to keep them in memory.
The create-3 test run seems to “cheat” when using borg. Not reading any data doesn’t make sense, unless borg is just getting all data it needs from the server.