Design and Implementation of a Direct Access File System (DAFS) Kernel Server for FreeBSD

The Direct Access File System (DAFS) is an emerging commercial standard for network-attached storage on server cluster interconnects. The DAFS architecture and protocol leverage network interface controller (NIC) support for user-level networking, remote direct memory access, efficient event notification, and reliable communication. This paper describes the design of the first implementation of a DAFS kernel server for FreeBSD, using existing interfaces with minor kernel modifications. We experimentally demonstrate that the current server structure can attain read throughput of more than 100 MB/s over a 1.25 Gb/s network even for small (i.e. 4K) block sizes when prefetching using an asynchronous client API. To reduce multithreading overhead and integrate the NIC with the host virtual memory system, our forthcoming system will incorporate new FreeBSD kernel support for asynchronous vnode I/O interfaces, integrating network and disk event notification and handling, and VM support for remote direct memory access. We believe our proposed kernel support is necessary to scale event-driven file servers to multi-gigabit network speeds.