On Wed, Jul 8, 2020 at 11:22 AM Dominique Martinet <asmadeus@codewreck.org> wrote:
Please test, but if a file is deemed not compressible (based on, not
sure what? the first few blocks?) then it will be stored in the
non-compressed version.
You can check with compsize after the fact if the file had been
compressed or not.

This should be true unless the compress-force mount option is used, even
the chattr is only a hint

OK, this is an interesting piece of information I wasn't aware of. In this case, if the btrfs heuristics work OK in the majority of cases, the performance hit might not be there, as I feared. Some thorough investigation would be needed, though.