Compression levels does not apply for bzip2, xz, zstd, lz4, now we map then as best as we can.

internal/backer/xz.go

@@ -10,6 +10,8 @@ import (
 // CreateTarXzStream takes a list of files and returns a reader that reads from an xz-compressed tar archive.
 func CreateTarXzStream(ctx context.Context, filepaths []string) io.ReadCloser {
 	return createArchiveStream(ctx, filepaths, func(w io.Writer) (io.WriteCloser, error) {
+		// xz doesn't have a simple level parameter; use default compression.
+		// The compression level affects block size indirectly through the algorithm.
 		return xz.NewWriter(w)
 	})
 }