R. Y. Wang, T. E. Anderson, D. A. Patterson.
Virtual Log Based File Systems for a Programmable Disk.
Proc. Third Symposium on Operating Systems Design and
Implementation.
February 1999.
Also appeared as University of California Technical Report CSD-98-1031.
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.