On *NIX platforms the statfs(2) system call returns a struct containing both the
free blocks in the filesystem (Statfs_t.Bfree) and the free blocks available to
the unprivileged or non-superuser (Statfs_t.Bavail).
The `Bfree` and `Bavail` fields (with `Bfree >= Bavail`) will be set to
different values on e.g. filesystems such as ext4 that reserve a certain
percentage of the filesystem blocks which may only be allocated by admnistrative
privileged processes.
The calculations for the `Total` disk space need to subtract the difference
between the `Bfree` and `Bavail` fields for it to correctly show the total
available storage space available for unprivileged users.
This implicitly fixes a bug where the `Used = Total - Free` calculation yielded
different (and also incorrect) results for identical contents stored when only
the sizes of the disks or backing volumes differed. (as can be witnessed in the
`Used:` value displayed in the Minio browser)
See:
- https://wiki.archlinux.org/index.php/ext4#Reserved_blocks
- http://man7.org/linux/man-pages/man2/statfs.2.html
- https://man.openbsd.org/statfs
- http://lingrok.org/xref/coreutils/src/df.c#893
