Chapter 1: Overview

Section 1.1: Introduction

This document describes the architecture and interfaces for two of the important agents of the AFS distributed file system, the Volume Server and the Volume Location Server. The Volume Server allows operations affecting entire AFS volumes to be executed, while the Volume Location Server provides a lookup service for volumes, identifying the server or set of servers on which volume instances reside.

Section 1.2: Volumes

Section 1.2.1: Definition

The underlying concept manipulated by the two AFS servers examined by this document is the volume. Volumes are the basic mechanism for organizing the data stored within the file system. They provide the foundation for addressing, storing, and accessing file data, along with serving as the administrative units for replication, backup, quotas, and data motion between File Servers.
Specifically, a volume is a container for a hierarchy of files, a connected file system subtree. In this respect, a volume is much like a traditional unix file system partition. Like a partition, a volume can be mounted in the sense that the root directory of the volume can be named within another volume at an AFS mount point. The entire file system hierarchy is built up in this manner, using mount points to glue together the individual subtrees resident within each volume. The root of this hierarchy is then mounted by each AFS client machine using a conventional unix mount point within the workstation's local file system. By convention, this entryway into the AFS domain is mounted on the /afs local directory. From a user's point of view, there is only a single mount point to the system; the internal mount points are generally transparent.

Section 1.2.2: Volume Naming

There are two methods by which volumes may be named. The first is via a human-readable string name, and the second is via a 32-bit numerical identifier. Volume identifiers, whether string or numerical, must be unique within any given cell. AFS mount points may use either representation to specify the volume whose root directory is to be accessed at the given position. Internally, however, AFS agents use the numerical form of identification exclusively, having to translate names to the corresponding 32-bit value.

Section 1.2.3: Volume Types

There are three basic volume types: read-write, read-only, and backup volumes.

Section 1.3: Scope

This paper is a member of a documentation suite providing specifications of the operation and interfaces offered by the various AFS servers and agents. The scope of this work is to provide readers with a sufficiently detailed description of the Volume Location Server and the Volume Server so that they may construct client applications which call their RPC interface routines.

Section 1.4: Document Layout

After this introductory portion of the document, Chapters 2 and 3 examine the architecture and RPC interface of the Volume Location Server and its replicated database. Similarly, Chapters 4 and 5 describe the architecture and RPC interface of the Volume Server.

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