Virtual Log Based File Systems for a Programmable Disk

In this paper, we study how to minimize the latency of small synchronous writes to disks. The basic approach is to write to free sectors that are near the current disk head location by leveraging the embedded processor core inside the disk. We develop a number of analytical models to demonstrate the performance potential of this approach. We then present the design of a virtual log, a log whose entries are not physically contiguous, and a variation of the log-structured file system based on this approach. The virtual log based file systems can efficiently support small synchronous writes without extra hardware support while retaining the advantages of LFS including its potential to support transactional semantics. We compare our approach against traditional update-in-place and logging systems by modifying the Solaris kernel to serve as a simulation engine. Our evaluations show that random synchronous updates on an unmodified UFS execute up to an order of magnitude faster on a virtual log than on a conventional disk. The virtual log can also significantly improve LFS in cases where delaying small writes is not an option or on-line cleaning would degrade performance. If the current trends of disk technology continue, we expect the performance advantage of this approach to become even more pronounced in the future.