The total size is the amount of bytes which could be written, using the definition from my above comment, if the filesystem were *empty*. The used size is (total - free).
This would have the following desirable properties:
- A freshly formatted filesystem would have "used" 0 and "free" = "total".
- A completely full filesystem would have "free" 0 and "used" = "total".
- Adding new files is monotonic; "used" increases (or stays the same), "free" decreases (or stays the same).
- The "total" value does not change, no matter how many files you add or remove.
- If "free" says you have x bytes, you can create at least a file of up to x bytes, even if its contents came from /dev/urandom.
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