Is "df" broken?

I have an external USB drive I use for backups, and I'm preparing to swap it out for a fresh one. It was full, and I'm been cleaning it a bit before the swap. In doing so I've found that

# df -B 1 <mount_point>

was producing:

1B-blocks 1968842792960
Used 1870698692608
Available 0
Use% 100%

That surprises me a little ...

Ubuntu 20.04 LTS

# df --version
df (GNU coreutils) 8.28

Comments?

@ColinTheMathmo What file system on that drive? If it's an ext* fs, could it be a combo of reserved blocks and fs overhead? The total blocks is definitely raw blocks, not what's available to the fs, just checking on a drive that I have and adding the used/avail up.

@cefiar It's EXT4 ... I'm finding that the Usage number goes up and down exactly as expected. Archive files to a tarball, the "Used" figure goes up. Remove the originals, the "Used" figure goes down.

The "Available" and "Use%" figures remain static at 0 and 100% respectively, even when subtracting or dividing "Used" and "Total" appropriately show that they should not be.

It's as if a "needle" got to the limit of its travel and got stuck there.

@ColinTheMathmo @cefiar are you writing to the disk as root? That lets you override the 'reserved block'

@penguin42 @cefiar Ah, good point ... (pause) ... OK, trying to write as user gives me "no space on device" but writing as sudo let's me succeed.

OK, so how do I return to getting useful results from df?

(and thank you)

@ColinTheMathmo @cefiar The answer depends what you want; I think if you run the df as root it'll give you the real answer; as a normal user it's telling you what is available to use. If you just want it to let you use all the disk as a normal user then use tune2fs -m 0 to remove the reserved blocks.

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@penguin42 @cefiar Running "df" as root also gives me 0 and 100% for Available and Use%, so that seems not to be the entire story.

I'll think about "tune2fs -m 0" ... thanks.

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