I've tested the issue packing files in the range of 10s of GB with 7z compression ranging from fast to ultra, but on 7.7.1 and new 7.8.0 release it seems there is no anomalous space occupation outside the intended working directory.
C is usually the system disk, and it is possible other processes are writing to it, possibly the system may be writing paging on disk because at high compression levels 7z may be very memory intensive.
I'll test more cases to see if the issue you are reporting is reproducible, please feel free to add any further information you find relevant to replicate the problem.
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I found no cases in which temporary work files were saved on system disk if specified elsewhere, please re-open the issue if you find a reproducible case to test.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
I've tested the issue packing files in the range of 10s of GB with 7z compression ranging from fast to ultra, but on 7.7.1 and new 7.8.0 release it seems there is no anomalous space occupation outside the intended working directory.
C is usually the system disk, and it is possible other processes are writing to it, possibly the system may be writing paging on disk because at high compression levels 7z may be very memory intensive.
I'll test more cases to see if the issue you are reporting is reproducible, please feel free to add any further information you find relevant to replicate the problem.
I found no cases in which temporary work files were saved on system disk if specified elsewhere, please re-open the issue if you find a reproducible case to test.