About the file compression block size

The file compression algorithm compresses data in the specified block size, which defaults to 1MB. Each compression block has its own extent descriptor in the inode. If the file or the last extent is smaller than the compression block size, then that smaller size gets compressed. The maximum block size is 1MB.

Extents with data that cannot be compressed are still marked as compressed extents. Even though such extents cannot be compressed, marking these extents as compressed allows successive compression runs to skip these extents to save time. Shared extents cannot be compressed and do not get marked as compressed. Since the file compression algorithm looks at fixed-size blocks, the algorithm finds these incompressible extents in units of the file compression block size.