Inline data is compressed unconditinally even when results in increased size

Inline data is compressed unconditinally even when results in increased size

[Hanabishi]

Hello.

I noticed that a btrfs volume with compression enabled (e.g. 'compress=zstd') seems to compress inline data unconditionally, even in situations when compression is ineffective and only leads to increase in size.

It is easily reproducible on small files:

# echo 'a file with test content' > test.txt
# sync
# compsize test.txt
Processed 1 file, 0 regular extents (0 refs), 1 inline.
Type       Perc     Disk Usage   Uncompressed Referenced
TOTAL      172%       43B          25B          25B
zstd       172%       43B          25B          25B

Same file when no compression enabled:

Processed 1 file, 0 regular extents (0 refs), 1 inline.
Type       Perc     Disk Usage   Uncompressed Referenced
TOTAL      100%       25B          25B          25B
none       100%       25B          25B          25B

It's not a big deal on practice, but shouldn't it detect that the file is uncompressible and fall back to no compression?
Also, if I remember correctly, it wasn't the case a couple of kernel releases back.

Re: Inline data is compressed unconditinally even when results in increased size

[Filipe Manana]

On Mon, Sep 14, 2026 at 5:53 PM Hanabishi
<i.r.e.c.c.a.k.u.n+kernel.org@gmail.com> wrote:
>
> Hello.
>
> I noticed that a btrfs volume with compression enabled (e.g. 'compress=zstd') seems to compress inline data unconditionally, even in situations when compression is ineffective and only leads to increase in size.
>
> It is easily reproducible on small files:
>
> # echo 'a file with test content' > test.txt
> # sync
> # compsize test.txt
> Processed 1 file, 0 regular extents (0 refs), 1 inline.
> Type       Perc     Disk Usage   Uncompressed Referenced
> TOTAL      172%       43B          25B          25B
> zstd       172%       43B          25B          25B
>
> Same file when no compression enabled:
>
> Processed 1 file, 0 regular extents (0 refs), 1 inline.
> Type       Perc     Disk Usage   Uncompressed Referenced
> TOTAL      100%       25B          25B          25B
> none       100%       25B          25B          25B
>
> It's not a big deal on practice, but shouldn't it detect that the file is uncompressible and fall back to no compression?
> Also, if I remember correctly, it wasn't the case a couple of kernel releases back.

It's a recent regression, independent of the compression algorithm used.
Fix here:

https://lore.kernel.org/linux-btrfs/721d8fd2c73095d06cbdbfa714a6dfac9c488953.1789407346.git.fdmanana@suse.com/

Thanks.

>
>

Re: Inline data is compressed unconditinally even when results in increased size

[Hanabishi]

> It's a recent regression, independent of the compression algorithm used.
Good that I remember it right, I wasn't sure.
By the way, the actually worst case scenario when it's just 1 byte.

Processed 1 file, 0 regular extents (0 refs), 1 inline.
Type       Perc     Disk Usage   Uncompressed Referenced
TOTAL      1900%      19B           1B           1B
zstd       1900%      19B           1B           1B