HAMMER2 File-System Is Still Slowly Coming Together
As we haven't heard much about HAMMER2 in quite some times, I was looking through the DragonFlyBSD HAMMER2 Git commits and happy to see this BTree-based file-system succeeding the original HAMMER FS is still being pursued. HAMMER2 has shipped in DragonFlyBSD since version 3.8.0 but is not considered production ready.
Matthew Dillon continues working on HAMMER2 but over 2014 there's been times when there's been n public commits over two month blocks. The technical plans for the HAMMER2 file-system are outlined in the aforelinked article talking about the open-source file-system's original announcement.
Those curious about the current state of HAMMER2 can see the Git-based TODO list for the items still outstanding. Outside of the DragonFlyBSD world, there hasn't been much interest in HAMMER2 since other BSDs have centered around the ZFS file-system. For Linux systems, there's OpenZFS along with Btrfs and other alternatives like F2FS.