2.2.5. ATM Switching. end;. The culprit here is that although there is a global time ordering within each group, there is not necessarily any coordination among multiple groups. Some systems support well-defined time ordering among overlapping groups and others do not. (If the groups are disjoint, the issue does not arise.) Implementing time ordering among different groups is frequently difficult to do, so the question arises as to whether it is worth it.Scalability. 4. Durable: Once a transaction commits, the changes are permanent.. A files can haveattributes,which are pieces of information about the file but which are not part of the file itself. Typical attributes are the owner, size, creation date, and access permissions. The file service usually provides primitives to read and write some of the attributes. For example, it may be possible to change the access permissions but not the size (other than by appending data to the file). In a few advanced systems, it may be possible to create and manipulate user-defined attributes in addition to the standard ones.. For efficiency reasons, transfers between client and server are done in large chunks, normally 8192 bytes, even if fewer bytes are requested. After the client’s VFS layer has gotten the 8K chunk it needs, it automatically issues a request for the next chunk, so it will have it should it be needed shortly. This feature, known asread ahead,improves performance considerably.. Writes that are potentially causally related must be seen by allprocesses in the same order. Concurrent writes may be seen in a different order on different machines. . end;. All the threads in a process share the address space and all the process-wide resources shown in Fig. 8-2. Nevertheless, threads also have private per-thread resources. One of these is thethread port,which is analogous to the process port. each thread has its own thread port, which it uses to invoke thread-specific kernel services, such as exiting when the thread is finished. Since ports are process-wide resources, each thread has access to its siblings’ ports, so each thread can control the others if need be.. The goal of communication in Mach is to support a variety of styles of communication in a reliable and flexible way (Draves, 1990). It can handle asynchronous message passing, RPC, byte streams, and other forms as well. Mach’s interprocess communication mechanism is based on that of its ancestors, RIG and Accent. Due to this evolution, the mechanism used has been optimized for the local case (one node) rather than the remote case (distributed system).. Ports support reliable, sequenced, message streams. If a thread sends a message to a port, the system guarantees that it will be delivered. Messages are never lost due to errors, overflow, or other causes (at least if there are no crashes). Messages sent by a single thread are also guaranteed to be delivered in the order sent. If two threads write to the same port in an interleaved fashion, taking turns, the system does not provide any guarantee about message sequencing, since some buffering may take place in the kernel due to locking and other factors.. The last two header fields are not used by the kernel. Higher levels of software can use them as desired. By convention, they are used to specify the kind of message and give a function code or operation code (e.g., to a server, is this request for reading or for writing?). This usage is subject to change in the future.. It is probably worthwhile to make some of the assumptions underlying this model explicit, to avoid any confusion. We assume that the network is totally insecure, and that a determined intruder can capture any message sent on it, possibly removing it as well. The intruder can also inject arbitrary new messages and replay old ones to his heart’s content.. 10.7.2. DFS Components in the Server Kernel. Each entry in the data base contains the name of the fileset, the type (read/write, read-only, or backup), the number of servers holding it, the addresses of these servers, the fileset’s owner and group information, information about clones, cache timeout information, token timeout information, and other administrative information..