Sawmill: A High Bandwidth Logging File System

This paper describes the implementation of Sawmill, a network file system using the RAID-II storage system. Sawmill takes advantage of the direct data path in RAID-II between the disks and the network, which bypasses the file server CPU. The key ideas in the implementation of Sawmill are combining logging (LFS) with RAID to obtain fast small writes, using new log layout techniques to improve bandwidth, and pipelining through the controller memory to reduce latency. The file system can currently read data at 21 MB/s and write data at 15 MB/s, close to the raw disk array bandwidth, while running on a relatively slow Sun-4. Performance measurements show that LFS improved performance of a stream of small writes by over a order of magnitude compared to writing directly to the RAID, and this improvement would be even larger with a faster CPU. Sawmill demonstrates that by using a storage system with a direct data path, a file system can provide data at bandwidths much higher than the file server itself could handle. However, processor speed is still an important factor, especially when handling many small requests in parallel.