Determining the compression level used

[Nihon-Ryori]

Feature request: Determining the compression level used

Determining the compression level used for compression
In this context, I understand compression level to mean the question of which compression level was used to compress a file, e.g. with zstd in the possible range from -15 to -15.

Possible application:

The feature in question could be used as a criterion for subsequent higher compression to determine whether the file in question has already been compressed with the desired compression level. At the moment, the only workaround is to set files to be compressed via fstab, e.g. using lzo, and then check afterwards whether a file has only been compressed with lzo and, based on this criterion, compress it afterwards with zstd:5 or similar.

[Zygo]

This looks like the same request as #56, or at least it's close enough that the same approach works.

[Nihon-Ryori]

Ahh, compsize currently cannot implement output of the level used for compression (for zstd, e.g., in the range from -15 to +15), as BTRFS does not yet store the level used for compression.