This paper describes the changes from the original 512 byte UNIX file system to the new one released with the 4.2 Berkeley Software Distribution. It presents the motivations for the changes, the methods used to effect these changes, the rationale behind the design decisions, and a description of the new implementation. This discussion is followed by a summary of the results that have been obtained, directions for future work, and the additions and changes that have been made to the facilities that are available to programmers.
The original UNIX system that runs on the PDP-11** has simple and elegant file system facilities. File system input/output is buffered by the kernel; there are no alignment constraints on data transfers and all operations are made to appear synchronous. All transfers to the disk are in 512 byte blocks, which can be placed arbitrarily within the data area of the file system. Virtually no constraints other than available disk space are placed on file growth [Ritchie74], [Thompson78].*
When used on the VAX-11 together with other UNIX enhancements, the original 512 byte UNIX file system is incapable of providing the data throughput rates that many applications require. For example, applications such as VLSI design and image processing do a small amount of processing on a large quantities of data and need to have a high throughput from the file system. High throughput rates are also needed by programs that map files from the file system into large virtual address spaces. Paging data in and out of the file system is likely to occur frequently [Ferrin82b]. This requires a file system providing higher bandwidth than the original 512 byte UNIX one that provides only about two percent of the maximum disk bandwidth or about 20 kilobytes per second per arm [White80], [Smith81b].
Modifications have been made to the UNIX file system to improve its performance. Since the UNIX file system interface is well understood and not inherently slow, this development retained the abstraction and simply changed the underlying implementation to increase its throughput. Consequently, users of the system have not been faced with massive software conversion.
Problems with file system performance have been dealt with extensively in the literature; see [Smith81a] for a survey. Previous work to improve the UNIX file system performance has been done by [Ferrin82a]. The UNIX operating system drew many of its ideas from Multics, a large, high performance operating system [Feiertag71]. Other work includes Hydra [Almes78], Spice [Thompson80], and a file system for a LISP environment [Symbolics81]. A good introduction to the physical latencies of disks is described in [Pechura83].