Re: Disk usage difference in the output of `btrfs fi show` vs. `btrfs fi du`

[Andrei Borzenkov <arvidjaar@gmail.com>]

On 28.03.2021 15:28, Zoltán wrote:
> Hi,
> 
> I have read a lot of caveats about interpreting the free space
> reported for btrfs volumes, but could not find anything about the
> perceived inconsistency in the disk usage reporting described below.
> 
> I have a btrfs volume with about 135GiB used for data, as reported by
> `df`, `btrfs fi show` and `btrfs fi usage` alike:
> 
> # btrfs filesystem show /volumes/main/
> Label: 'main'  uuid: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
>         Total devices 1 FS bytes used 134.12GiB
>         devid    1 size 193.43GiB used 165.01GiB path /dev/sda2
> 
> However, `btrfs fi du` reports ~17GiB exclusive usage and ~80GiB
> shared usage, which adds up to only 97GiB (compared to the 135GiB I
> would expect):
> 
> # btrfs filesystem du -s /volumes/main/
>      Total   Exclusive  Set shared  Filename
>    1.73TiB    17.20GiB    80.13GiB  /volumes/main/
> 
> (The reported total usage exceeding the disk capacity by an order of
> magnitude is expected as the volume contains many snapshots.)
> 
> The mount point corresponds to the root subvolume, thus all subvolumes
> should be accounted for by `btrfs fi du` (according to its
> documentation):
> 
> # mount | grep /volumes/main
> /dev/sda2 on /volumes/main type btrfs
> (rw,noatime,ssd,space_cache,autodefrag,subvolid=5,subvol=/)
> 
> Do I misunderstand the meaning of exclusive and shared usage or is
> there some other issue causing this behaviour? I would expect the disk

It probably does not take in account exclusive usage of other
subvolumes, likely snapshots.

> usage reported by `df`, `btrfs fi show` and `btrfs fi usage` to be the
> sum of the exclusive and shared usage reported by `btrfs fi du`.
> 
> The output of `btrfs fi usage`, for completeness's sake:
> 
> # btrfs filesystem usage /volumes/main/
> Overall:
>     Device size:                 193.43GiB
>     Device allocated:            165.01GiB
>     Device unallocated:           28.41GiB
>     Device missing:                  0.00B
>     Used:                        134.12GiB
>     Free (estimated):             57.00GiB      (min: 57.00GiB)
>     Data ratio:                       1.00
>     Metadata ratio:                   1.00
>     Global reserve:              194.72MiB      (used: 0.00B)
> 
> Data,single: Size:162.00GiB, Used:133.41GiB (82.35%)
>    /dev/sda2                     162.00GiB
> 
> Metadata,single: Size:3.01GiB, Used:720.88MiB (23.41%)
>    /dev/sda2                       3.01GiB
> 
> System,single: Size:4.00MiB, Used:48.00KiB (1.17%)
>    /dev/sda2                       4.00MiB
> 
> Unallocated:
>    /dev/sda2                      28.41GiB
> 
> Thanks,
> 
> Zoltan
>