DragonFly includes many useful features that differentiate it from other operating systems in the same class. The most prominent one is HAMMER, our modern high performance filesystem with built-in mirroring and historic access functionality. Virtual kernels provide the ability to run a full-blown kernel as a user process for the purpose of managing resources or for accelerated kernel development and debugging. The kernel makes extensive use of tokens as a synchronization mechanism; tokens are inherently deadlock-free and easily composable. The use of soft token locks results in less cross-subsystem pollution and more maintainable code, both of which allow us to parallelize the system with less effort compared to other kernels, which primarily use hard mutex locks. DragonFly is uniquely positioned to take advantage of the wide availability of affordable Solid Storage Devices (SSDs), by making use of swap space to cache filesystem data and meta-data. This feature, commonly referred to as "swapcache", can give a significant boost to both server and workstation workloads, with a very minor hardware investment.
The DragonFly storage stack comprises robust AHCI drivers, stable device names via DEVFS and a partial implementation of Device Mapper for reliable volume management and encryption. Some other features that are especially useful to system administrators are variant symlinks (i.e. symlinks that are resolved at runtime based on user-specific or system-wide variables) and a performant and scalable TMPFS implementation. Our system makes pervasive use of NULLFS mounts, which allow the administrator to make arbitrary parts of the filesystem hierarchy visible in other locations with virtually no overhead.