> From: Lassi Kortela <xxxxxx@lassi.io>
> Date: Tuesday, September 15, 2020 2:31 PM
>
>>>> I was writing some flaky tests to see if it's returning true results,
>>>> but the more I think of my assumptions, the harder this gets.  For
>>>> example, is there any filesystem almost guaranteed to be quiescent
>>>> between two calls of it?  /tmp is both iffy and not guaranteed to be a
>>>> separate file system, neither is /.  But maybe adding 32 MiB to
>>>> whatever is /tmp and checking to see if there's less free space is safe
>>>> enough.
>>>>
>>>> Advice is solicited; I have checked the results by hand with df, Linux
>>>> is fine, but either I'm too out of it, or OpenBSD is lying.
>>>
>>> Good questions.
>>
>> Let me ask another: is 32 GiB too much, too little, or about right?
>
> Sorry, I don't think I understood the line of reasoning.
>
> Are you writing a unit test to check whether the results are in the same
> ballpark if you get the free space multiple times for the same pathname?
> And wondering about what size ballpark to use :)

Not even "ballpark", although I suppose I should try for that.  Right
now, just a minimal sanity check, write 32 MiB, flush the port, make
sure the free-space is smaller than it was before I started writing to
/tmp.  And John's EC2 example of 10GiB for all of / including /tmp
hopefully wouldn't fail with this "small" a write.  But if so, it
could probably be moderately safely if smaller.

> I don't think any file system other than some kind of null mount or a
> read-only mount is guaranteed to stay the same size....

Indeed, I'm just spitballing how to try to find one that's most likely
to stay the same with two back to back calls to free-space.

> [...]

- Harold