Still dont know what lesson I should learn from this story.
-m reserved-blocks-percentage Set the percentage of the filesystem which may only be allocated by privileged processes. Reserving some number of filesystem blocks for use by privi‐ leged processes is done to avoid filesystem fragmentation, and to allow system daemons, such as syslogd(8), to continue to function correctly after non- privileged processes are prevented from writing to the filesystem. Normally, the default percentage of reserved blocks is 5%.
-g group Set the group which can use the reserved filesystem blocks. The group parameter can be a numerical gid or a group name. If a group name is given, it is converted to a numerical gid before it is stored in the superblock.
> when a process running as root runs df (or any similar command or syscall), then it sees that capacity as available.
I have never seen that. I have only ever seen the “real” available space being shown by tune2fs, not df or anything similar, which have always shown the space available after subtracting the reserved space.