Very interesting idea of having FS-level compression kicking in automatically on CouchDB compressions from Victor Igumnov:
CouchDB was made for next generation filesystems such as ZFS and BTRFS. First off, unlike PostgreSQL or MySQL, CouchDB can be snapshot while in production without any flushing or locking trickery since it uses an append only B-Tree storage approach. That alone makes it a compelling database choice on ZFS/BTRFS.
Second, CouchDB works hand-in-hand with ZFS’s block level compression. ZFS can compress blocks of data as they are being written out to the disk. However, it only does it for new blocks and not retroactively. Now, the awesome part, CouchDB on compaction writes out a brand new database file which can utilize the new gzip compression settings on ZFS. This means you can try out different gzip compression settings just by compacting your CouchDB.