Edward Shishkin, the former Namesys developer largely leading Reiser4's development for the past several years, announced in mid-March the Reiser4 file-system gained support for multiple transaction models. This news slipped under my radar until now. As a unique feature to Reiser4, the file-system can switch between different transaction models at mount-time.
All major file-systems are generally designed around one transaction model, such as being a journaling design or a write-anywhere / copy-on-write model, but Reiser4 now has support for swapping out the different transaction models. This support for multiple file-system transaction models was done since some modes work better for different drives; i.e. solid-state drives versus hard drives.
With the newest Reiser4 code, there's mount options for a journalling mode, a write-anywhere / copy-on-write model, and a hybrid transaction model. The hybrid model is what's been the default approach of Reiser4 going back to around 2002. Only the default hybrid mode is what's considered stable right now within the Reiser4 code.
Those open-source file-system enthusiasts wishing to learn more or to try out the new file-system code can find the details via Shishkin's mailing list announcement.