Chapter 4. I/O devices and file systems 155
Time Service DFS uses the DCE Distributed Time Service. AFS clients
use their cache manager and NTP to synchronize with the
AFS servers.
Synchronization Both DFS and AFS use a token manager to coordinate
concurrent access to the file system. However, AFS
revokes tokens from other clients when closing a file;
whereas, DFS already revokes the token when opening
the file. This means that DFS semantics are completely
conforming with local file system semantics, whereas,
AFS semantics are not. Nevertheless, AFS
synchronization is better than in NFS, which does not use
tokens at all.
It is obvious that DFS is well integrated with the other DCE core services;
whereas, AFS requires more configuration and administration work. DFS also
provides file system semantics that are superior to AFS. So, unless an existing
AFS cell is expanded, we recommend that DFS is used rather than AFS to
provide global file services.
4.5 Related documentation
This documentation will help you better understand the concepts and examples
covered in this chapter. We recommend that you take a look at some of these
books in order to maximize your chances of success in the SP certification exam.
SP manuals
RS/6000 SP Planning, Volume 2: Control Workstation and Software
Environment, GA22-7281. This manual gives detailed explanations on I/O
devices.
RS/6000 SP: Planning, Volume 1, Hardware and Physical Environment,
GA22-7280. This book is the official document for supported I/O adapters.
SP redbooks
Inside the RS/6000 SP, SG24-5145. NFS and AFS concepts are discussed in
this redbook.
IBM (e)server Cluster 1600 Managed by PSSP 3.5: What’s New, SG24-6617.

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