Like Btrfs, HAMMER2 supports transparent file-system compression. With this commit by Matthew Dillon, there's now LZ4 enabled by default whenever creating a new PFS (Psuedo File-System). Previously there was no compression by default. This change doesn't affect existing HAMMER2 setups.
Besides saving disk space, it's possible to yield faster performance too depending upon the file contents. For some benchmarks I did a few years ago, there's some old Btrfs compression numbers. I haven't yet run any HAMMER2 file-system benchmarks but will do so on DragonFlyBSD once the file-system has stabilized or is nearing its rollout by default.