Efficient User-Level File Cache Management on the Sun Vnode Interface

In developing a distributed file system, there are several good reasons for implementing the client file cache manager as a user-level process. These include ease of implementation, increased portability, and minimal impact on kernel size. For reasons of compatibility it is also desirable to use a standard file intercept mechanism on the client. The Sun VFS/Vnode file system interface is such a standard. However, this interface is designed for kernel-based file systems, and a user-level cache manager that used the Vnode mechanism would pay a large performance penalty due to the high number of kernel to cache manager context switches per file system call. This paper describes our solution to the problem for the Coda file system. By using a relatively small amount of kernel code to cache critical information, we are able to retain the much larger and more complex components of the Coda cache manager in a user level process. The measurements of Coda presented here confirm the performance benefits of this strategy, and indicate the relative merits of caching different kinds of information in the kernel. This research was supported in part by the Defense Advanced Research Projects Agency (DOD) and monitored by the Avionics Laboratory, Air Force Wright Aeronautical Laboratories, Aeronautical Systems Division (AFSC), Wright-Patterson AFB, Ohio 45433-6543 under Contract F33615-87-C-1499, ARPA Order No. 4976, Amendment 20 and in part by the National Science Foundation under Contract No. CCR-8657907, IBM Corporation (Faculty Development Award and Graduate Fellowship), Digital Equipment Corporation (Equipment Grant). The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of DARPA, the National Science Foundation, IBM Corporation, Digital Equipment Corporation or the U.S. government.