Re: 100% disk usage reported by "df", 60% disk usage reported by "btrfs fi usage" - this breaks userspace behaviour

[Chris Murphy <lists@colorremedies.com>]

On Thu, Dec 5, 2019 at 7:04 PM Tomasz Chmielewski <mangoo@wpkg.org> wrote:
>
> I know that "df -h" is not to be trusted with btrfs, but some userspace
> tools (i.e. apt, monitoring checks) rely on it.
>
> Sometimes, "df -h" will report 100% disk usage on a btrfs filesystem,
> while there is still space available and writes are possible.
>
> Linux 5.4.1, LXD containers located in /data:
>
> # df -h
> /dev/nvme1n1               1000G  571G     0 100% /data
>
> # btrfs fi usage /data
> Overall:
>      Device size:                1000.00GiB
>      Device allocated:            580.02GiB
>      Device unallocated:          419.98GiB
>      Device missing:                  0.00B
>      Used:                        570.06GiB
>      Free (estimated):            429.45GiB      (min: 429.45GiB)
>      Data ratio:                       1.00
>      Metadata ratio:                   1.00
>      Global reserve:              512.00MiB      (used: 0.00B)


It does look like a bug. But I'm not sure if this is expected, I
thought such significant disagreements were long since fixed.

What version of coreutils?

Maybe attach a strace of df? (I'm not sure of the list attach size
limit but it's preferred, but something like a pastebin is OK also)

And maybe this will be useful for a dev?
# grep -r . /sys/fs/btrfs/<fs-uuid>/allocation/

You'll need to use the correct fs uuid for the btrfs in question.


> Data,single: Size:578.01GiB, Used:568.54GiB
>     /dev/nvme1n1  578.01GiB

df seems to be treating only unallocated space as free, rather than
both unallocated and allocated that is unused. Color me surprised.



-- 
Chris Murphy