The Andrew File System (AFS) University of （安德鲁文件系统(AFS)大学）.pdf

49 The Andrew File System (AFS) The Andrew File System was introduced at Carnegie-Mellon University (CMU) 1 in the 1980’s [H+88]. Led by the well-known Professor M. Satya- narayanan of Carnegie-Mellon University (“Satya” for short), the main goal of this project was simple: scale. Specifically, how can one design a distributed file system such that a server can support as many clients as possible? Interestingly, there are numerous aspects of design and implementa- tion that affect scalability. Most important is the design of the protocol be- tween clients and servers. In NFS, for example, the protocol forces clients to check with the server periodically to determine if cached contents have changed; because each check uses server resources (including CPU and network bandwidth), frequent checks like this will limit the number of clients a server can respond to and thus limit scalability. AFS also differs from NFS in that from the beginning, reasonable user- visible behavior was a first-class concern. In NFS, cache consistency is hard to describe because it depends directly on low-level implementa- tion details, including client-side cache timeout intervals. In AFS, cache consistency is simple and readily understood: when the file is opened, a client will generally receive the latest consistent copy from the server. 49.1 AFS Version 1 We will discuss two versions of AFS [H+88, S+85]. The first version (which we will call AFSv1, but actually the original system was called the ITC distributed file system [S+85]) had some of the basic design in place, but didn’t scale as desired, wh