tyt12fi.htm

tcpip 协议学习电子书籍 第一次上传东西

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<P>Read file

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<P>Reads a file

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<P>Write to cache

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<P>Not used

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<P>Write to file

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<P>Writes a file

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<P>Create file

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<P>Creates a new file and returns the new file's handle

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<P>Delete file

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<P>Deletes a file

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<P>Rename file

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<P>Renames a file

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<P>Generate link

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<P>Creates a link to a file (same file system)

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<P>Generate symbolic link

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<P>Generates a symbolic link (across file systems)

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<P>Create directory

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<P>Creates a new directory

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<P>Delete directory

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<P>Removes a directory

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<P>Read directory

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<P>Returns a list of files in the directory

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<P>Read file system attributes

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<P>Returns information about the file system</FONT>

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<P>Programmers might have noticed the lack of any open and close file functions within NFS. This arises from the stateless nature of the protocol. When a file must be opened, the local NFS process handles it, not the remote process. This allows for better control of files and ports after failure of a machine or a connection.

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<FONT SIZE=4 COLOR="#FF0000"><B>Mount Protocol</B></FONT></CENTER></H4>

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<P>In today's introduction I mentioned that NFS works by mounting an NFS server file system onto a client file system. As you have just seen, however, the NFS protocol is actually about file access and information, not connecting file systems. This file system mounting procedure is dealt with as a separate issue by the NFS product, using the Mount protocol. Mount uses UDP.

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<P>The Mount protocol is involved in returning a file handle from the server to the client, enabling the client to access an area on the server file system. The protocol returns not only the file handle but also the name of the file system in which the requested file resides. Mount consists of a number of procedures that facilitate communications between the client and server, designed especially for dealing with files.

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<P>A process called mountd takes care of handling the mount protocol at both ends of a connection. The mountd process maintains a list of machines and path names that are involved in a mount operation. Once a mount has been performed, NFS can continue operating without referring back to Mount at all. This lets Mount continue to modify its internal tables without affecting ongoing sessions. This can cause a problem if a client crashes and reconnects. The server still has the original connections listed in its internal Mount tables. To correct this problem, most NFS clients send a command (called UMNTALL, or unmount all) to all NFS servers when they boot.

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<IMG SRC="note.gif" tppabs="http://www.mcp.com/817948800/0-672/0-672-30885-1/note.gif" WIDTH = 75 HEIGHT = 46>The Mount protocol is involved only during the original connection between a client and a server. The mountd process keeps track of connections, but once the connection is established, the Mount protocol relinquishes all control to NFS. All NFS needs to access a file system is a valid file handle (which Mount provides when the connection is made).</NOTE>

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<P>As mentioned earlier, the Mount protocol consists of a set of procedures. These are summarized in Table 12.2 and are self-explanatory.

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<FONT COLOR="#000080"><B>Table 12.2. Mount protocol procedures.</B></FONT></CENTER>

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<P><B><I>Name</I></B>

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<P><B><I>Description</I></B>

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<P>NULL

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<P>Null

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<P>MNT

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<P>Mounts a file system and returns the file handle and file system name

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<P>UMNT

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<P>Unmounts a server file system, deleting the entry from the Mount table

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<P>UMNTALL

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<P>Unmounts all server file systems used by the client and updates the Mount table