LINUX-VFS的supper bolck、inode、dentry和file关系简述<转>

The common file model consists of the following object types:
<1>The superblock object
Stores information concerning a mounted filesystem. For disk-based filesystems, this object usually corresponds to a filesystem control block stored on disk.
<2>The inode object
Stores general information about a specific file. For disk-based filesystems, this object usually corresponds to a file control block stored on disk. Each inode object is associated with an inode number, which uniquely identifies the file within the filesystem.
<3>The file object
Stores information about the interaction between an open file and a process. This information exists only in kernel memory during the period when a process has the file open.
<4>The dentry object
Stores information about the linking of a directory entry (that is, a particular name of the file) with the corresponding file. Each disk-based filesystem stores this information in its own particular way on disk.

Figure 12-2 illustrates with a simple example how processes interact with files. Three different processes have opened the same file, two of them using the same hard link. In this case, each of the three processes uses its own file object, while only two dentry objects are requiredone for each hard link. Both dentry objects refer to the same inode object, which identifies the superblock object and, together with the latter, the common disk file.

Interaction between processes and VFS objects

需要注意的几点如下所示：

1）进程每打开一个文件，就会有一个file结构与之对应。同一个进程可以多次打开同一个文件而得到多个不同的file结构，file结构描述被打开文件的属性，如文件的当前偏移量等信息。

2）两个不同的file结构可以对应同一个dentry结构。进程多次打开同一个文件时，对应的只有一个dentry结构。Dentry结构存储目录项和对应文件（inode）的信息。

3）在存储介质中，每个文件对应唯一的inode结点，但是每个文件又可以有多个文件名。即可以通过不同的文件名访问同一个文件。这里多个文件名对应一个文件的关系在数据结构中表示就是dentry和inode的关系。

4）Inode中不存储文件的名字，它只存储节点号；而dentry则保存有名字和与其对应的节点号，所以就可以通过不同的dentry访问同一个inode。

5）不同的dentry则是同个文件链接（ln命令）来实现的。