tailieunhanh - modern operating systems 2nd edition phần 9

Được xuất bản bởi Microsoft Press Phòng của Tập đoàn Microsoft Microsoft Way Redmond, Washington 98052-6399 Bản quyền © 2004 bởi Tập đoàn Microsoft Tất cả các quyền. Không có phần nào của nội dung của cuốn sách này có thể được sao chép | 752 CASE STUDY 1 UNIX AND LINUX CHAP. Ỉ0 or remote. If it is local the correct file system and i-node can be located. If it is remote the remote host and file handle can be located. When a remote file is opened on the client at some point during the parsing of the path name the kernel hits the directory on which the remote file system is mounted. It sees that this directory is remote and in the directory s v-node finds the pointer to the r-node. it then asks the NFS client code to open the file. The NFS client code looks up the remaining portion of the path name on the remote server associated with the mounted directory and gets back a file handle for it. It makes an r-node for the remote file in its tables and reports back to the VFS layer which puts in its tables a v-node for the file that points to the r-node. Again here we see that every open file or directory has a v-node that points to either an r-node or an i-node. The caller is given a file descriptor for the remote file. This file descriptor is mapped onto the v-node by tables in the VFS layer. Note that no table entries are made on the server side. Although the server is prepared to provide file handies upon request it does not keep ưack of which files happen to have file handles outstanding and which do not. When a file handle is sent to it for file access it checks the handle and if it is valid uses it. Validation can include verifying an authentication key contained in the RPC headers if security is enabled. When the file descriptor is used in a subsequent system call for example read the VFS layer locates the corresponding v-node and from that determines whether it is local or remote and also which i-node or r-node describes it. It then sends a message to the server containing the handle the file offset which is maintained on the client side not the server side and the byte count. For efficiency reasons transfers between client and server are done in large chunks normally 8192 bytes even if fewer .