Distributed File Sytsem :: Essays Papers

Distributed File Sytsem Intro CODA is an experimental distributed file system being developed at Carnegie Mellon University. Dr. M. Satyanarayanan heads this group whose purpose is to design and implement a distributed file system that allows for transparent mobile computing in a client server environment. The goals set forth for CODA include disconnected operation for mobile clients, failure resistance, performance, scalability and security. Architecture Most of the aspects of CODA’s basic architecture are inherited from its predecessor AFS (Andrew File System). Like AFS, CODA makes a distinction between servers and clients. The CODA workstation servers consist of partitions available to the file server. The partitions are made up of volumes that contain files. The volumes are set up like a directory structure, a root directory and the tree below it. A typical server would have hundreds of volumes of around 10MB each. The use of volumes helps to make large amounts of data more manageable and flexible. CODA stores volume and directory information along with access control lists and file attributes in raw partitions. These partitions are accessed through a log based recoverable virtual memory package (RVM). Only the file data is stored in the server partition files. Each of the volumes has a name and an ID. Volumes can be mounted anywhere in /coda except for under existing directories. A new directory will be created as part of the mount process (the volume name cannot conflict with existing directories in order to eliminate confusion). CODA makes the mounting points invisible to the user; they appear as regular directories. Files in CODA are identified by a FID (File identifier). The Fid consists of three 32 bit integers: the VolumeId, a VnodeId and an Uniquifier. The VolumeId identifies the volume the file resides in, the VnodeId is the inode number of the file and the Uniquifier guarantees no FID is ever used more than once. CODA stores replicated copies of volumes on a group of servers for higher availability and reliability. The list of the servers that hold a copy of the replicated volume is the VSG or Volume Storage Group. Each replicated volume’s VolumeId is also replicated. The replicated VolumeIds keep the VSG and the local volumes of each member together. Connected Operation The inner workings of a file system operation in CODA begin in much the same way they would in many other file systems.