A quick one.

Many (most?) filesystems in Linux can define reserved space. This is space that only the superuser can write to, and 
it's usually reserved for times when you've run out of space, allowing the super user to still operate the computer,
even when normal users can't write anymore. This is also why there is the concept of available vs free space in 
partitions.

Historically, reserved space was defined in percentage. But with bigger and bigger disks (30TB now?), even 1%
is a _a lot_ of space (300GB). Fortunately, we can also define it in blocks. At 4KiB per block, you can reserve "only"
1TiB with:

    tune2fs -r $((1024 * 1024 / 4)) [partition]
    
Unluckily, many of these tools don't define `--long-options`.