[dolphin] [Bug 473074] New: Refuses to copy a copyable folder with "not enough space on destination" on BTRFS with compression

[Martin Steigerwald]

https://bugs.kde.org/show_bug.cgi?id=473074

            Bug ID: 473074
           Summary: Refuses to copy a copyable folder with "not enough
                    space on destination" on BTRFS with compression
    Classification: Applications
           Product: dolphin
           Version: 22.12.3
          Platform: Other
                OS: Other
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: dolphin-bugs-null at kde.org
          Reporter: Martin at Lichtvoll.de
                CC: kfm-devel at kde.org
  Target Milestone: ---

SUMMARY

Dolphin refused to copy a folder with 101,7 GiB from zstd, level 3, compressed
BTRFS to another zstd, level 3, compressed BTRFS with about 99 GiB free space.

But the folder can still be copied cause in compressed form it only requires
about 35 GiB. So after the copy BTRFS still reports 62,3 GiB free.

I even checked beforehand that the copy can still succeed with the compsize
command that is available in Debian package btrfs-compsize. Unfortunately that
command requires superuser permissions to determine the compression state of
files. And it would only help in case the source is also a compressing
filesystem with same or at least similar compression levels.


STEPS TO REPRODUCE

1. Create a small filesystem with compression, like BTRFS with zstd.
2. Copy a folder with compressible files that is larger in total size to that
filesystem


OBSERVED RESULT

Dolphin refuses to copy the folder to the destination without giving the user
any option to override that behavior with some kind of "I know what I am
doing".


EXPECTED RESULT

Dolphin offers some copy it anyway option, probably with an explanation that
the copy could still succeed in case the filesystem compresses data.

Another idea would be to turn of that check in case KDE detects a compressing
filesystem.

A third would be to somehow determine whether the copy can succeed with
accounting for compression, but that would be challenging to get right. I think
it would require to test compress the data the way the filesystem does it.


SOFTWARE/OS VERSIONS

Linux: Devuan GNU/Linux 6 aka Daedalus with self-compiled 6.4.7 kernel.
KDE Plasma Version: 5.27.7
KDE Frameworks Version: 5.107.0
Qt Version: 5.15.10


ADDITIONAL INFORMATION

By default BTRFS does not compress. So that limits the scope of the issues to
users who activate compression by themselves.

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