The Coda Distributed File System
The Coda distributed file system is a state-of-the-art experimental file system developed in the group of M. Satyanarayanan at Carnegie Mellon University (CMU). Numerous people contributed to Coda, which now incorporates many features not found in other systems:
disconnected operation for mobile clients
reintegration of data from disconnected clients
read/write replication servers
resolution of server/server conflicts
handles network failures which partition the servers
handles disconnection of client's client
Performance and scalability:
client-side persistent caching of files, directories and attributes for high performance
access control lists (ACLs)
Well-defined semantics of sharing
Freely available source code
A distributed file system stores files on one or more computers called servers and makes them accessible to other computers called clients, where they appear as normal files. There are several advantages to using file servers: the files are more widely available since many computers can access the servers, and sharing the files from a single location is easier than distributing copies of files to individual clients. Backups and safety of the information are easier to arrange since only the servers need to be backed up. The servers can provide large storage space, which might be costly or impractical to supply to every client. The usefulness of a distributed file system becomes clear when considering a group of employees sharing documents; however, more is possible. For example, sharing application software is an equally good candidate. In both cases system administration becomes easier.
There are many problems facing the design of a good distributed file system. Transporting many files over the Net can easily create sluggish performance and latency; network bottlenecks and server overload can result. The security of data is another important issue: how can we be sure that a client is really authorized to have access to information and how can we prevent data being sniffed off the network? Two further problems facing the design are related to failures. Often, client computers are more reliable than the network connecting them, and network failures can render a client useless. Similarly, a server failure can be very unpleasant, since it can disable all clients from accessing crucial information. The Coda project has paid attention to many of these issues and implemented them as a research prototype.
Coda was originally implemented on Mach 2.6 and has recently been ported to Linux, NetBSD and FreeBSD. Michael Callahan ported a large portion of Coda to Windows 95, and we are studying Windows NT to understand the feasibility of porting Coda to NT. Currently, our efforts are on ports and on making the system more robust. A few new features are being implemented (write-back caching and cells for example), and in several areas, components of Coda are being reorganized. We have already received very generous help from users on the Net, and we hope that this will continue. Perhaps Coda can become a popular, widely used and freely available distributed file system.
If Coda is running on a client, which we shall take to be a Linux workstation, typing mount will show a file system—of type “Coda”--mounted under /coda. All the files, which any of the servers may provide to the client, are available under this directory, and all clients see the same name space. A client connects to “Coda” and not to individual servers, which come into play invisibly. This is quite different from mounting NFS file systems which is done on a per server, per export basis. In the most common Windows systems (Novell and Microsoft's CIFS) as well as with Appleshare on the Macintosh, files are also mounted per volume. Yet the global name space is not new. The Andrew file system, Coda's predecessor, pioneered the idea and stored all files under /afs. Similarly, the distributed file system DFS/DCE from OSF mounts its files under one directory. Microsoft's new distributed file system (dfs) provides glue to put all server shares in a single file tree, similar to the glue provided by auto-mount daemons and yellow pages on UNIX. Why is a single mount point advantageous? It means that all clients can be configured identically, and users will always see the same file tree. For large installations this is essential. With NFS, the client needs an up-to-date list of servers and their exported directories in /etc/fstab, while in Coda a client merely needs to know where to find the Coda root directory /coda. When new servers or shares are added, the client will discover these automatically in the /coda tree.
To understand how Coda can operate when the network connections to the server have been severed, let's analyze a simple file system operation. Suppose we type:
to display the contents of a Coda file. What actually happens? The cat program will make a few system calls in relation to the file. A system call is an operation through which a program asks the kernel for service. For example, when opening the file the kernel will want to do a lookup operation to find the inode of the file and return a file handle associated with the file to the program. The inode contains the information to access the data in the file and is used by the kernel; the file handle is for the opening program. The open call enters the virtual file system (VFS) in the kernel, and when it is realized that the request is for a file in the /coda file system, it is handed to the Coda file system module in the kernel. Coda is a fairly minimalistic file-system module: it keeps a cache of recently answered requests from the VFS, but otherwise passes the request on to the Coda cache manager, called Venus. Venus will check the client disk cache for tmp/foo, and in case of a cache miss, it contacts the servers to ask for tmp/foo. When the file has been located, Venus responds to the kernel, which in turn returns the calling program from the system call. Schematically we have the image shown in Figure 3.
The figure shows how a user program asks for service from the kernel through a system call. The kernel passes it up to Venus, by allowing Venus to read the request from the character device /dev/cfs0. Venus tries to answer the request, by looking in its cache, asking servers or possibly by declaring disconnection and servicing it in disconnected mode. Disconnected mode kicks in when there is no network connection to any server which has the files. Typically this happens for laptops when taken off the network or during network failures. If servers fail, disconnected operation can also come into action.
When the kernel passes the open request to Venus for the first time, Venus fetches the entire file from the servers, using remote procedure calls to reach the servers. It then stores the file as a container file in the cache area (currently /usr/coda/venus.cache/). The file is now an ordinary file on the local disk, and read/write operations to the file do not reach Venus but are (almost) entirely handled by the local file system (EXT2 for Linux). Coda read/write operations take place at the same speed as those to local files. If the file is opened a second time, it will not be fetched from the servers again, but the local copy will be available for use immediately. Directory files (remember, a directory is just a file) as well as all the attributes (ownership, permissions and size) are all cached by Venus, and Venus allows operations to proceed without contacting the server if the files are present in the cache. If the file has been modified and it is closed, Venus updates the servers by sending the new file. Other operations which modify the file system, such as making directories, removing files or directories and creating or removing (symbolic) links are propagated to the servers also.
So we see that Coda caches all the information it needs on the client, and only informs the server of updates made to the file system. Studies have confirmed that modifications are quite rare compared to “read only” access to files, hence we have gone a long way towards eliminating client-server communication. These mechanisms to aggressively cache data were implemented in AFS and DFS, but most other systems have more rudimentary caching. We will see later how Coda keeps files consistent, but first pursue what else one needs to support disconnected operation.
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