A key component in the Coda Distributed File System is the cache manager, Venus.
When processes on a Coda enabled system access files in the Coda filesystem, requests are directed at the filesystem layer in the operating system. The operating system will communicate with Venus to service the request for the process. Venus manages a persistent client cache and makes remote procedure calls to Coda file servers and related servers (such as authentication servers) to service these requests it receives from the operating system. When Venus has serviced a request it replies to the operating system with appropiate return codes, and other data related to the request. Optionally the kernel support for Coda may maintain a minicache of recently processed requests to limit the number of interactions with Venus. Venus possesses the facility to inform the kernel when elements from its minicache are no longer valid.
This document describes precisely this communication between the kernel and Venus. The definitions of so called upcalls and downcalls will be given with the format of the data they handle. We shall also describe the semantic invariants resulting from the calls.
Historically Coda was implemented in a BSD file system in Mach 2.6. The interface between the kernel and Venus is very similar to the BSD VFS interface. Similar functionality is provided, and the format of the parameters and returned data is very similar to the BSD VFS. This leads to an almost natural environment for implementing a kernel level filesystem driver for Coda in a BSD system. However, other operating systems such as Linux and Windows 95 and NT have virtual filesystem with different interfaces.
To implement Coda on these systems some reverse engineering of the Venus/Kernel protocol is necessary. Also it came to light that other systems could profit significantly from certain small optimizations and modifications to the protocol. To facilitate this work as well as to make future ports easier, communication between Venus and the kernel should be documented in great detail. This is the aim of this document.