In order to find a file referenced by a given pathname, each
of the components of the pathname must be read to find the
subsequent component. For example, take the file /etc/passwd
when used in a command such as:
In order to find the file passwd, the root directory (/) must first be found on the disk. Then the entry for the pathname component etc is used to locate that directory. The etc directory is read from the disk and used to locate the file passwd. The file passwd can then be read from the disk.
All of the above steps use Index Nodes or inodes. A file in a filesystem is represented by an inode which records its type, size, permissions, location, ownership, and access and modification dates. To locate the file's data, the inode also stores the block number (or numbers) of the disk blocks containing the data. Note that the inode does not contain the name of the file. Another file, a directory, stores the filename together with the corresponding inode number. In this way, several directory entries (or filenames) may refer to the same inode; these are known as hard links.
When a command accesses a pathname, such as /etc/passwd, the process of translating name to inode to data block has to be carried out for every component of the pathname before the file's data can be located. If a pathname component is a directory, such as /etc, the data blocks pointed to by its inode contain a map of filenames to inodes. This map is searched for the next pathname component, and this process continues until the final name component is reached. All inodes can be looked up in the inode table stored in memory, or if not present there, at the head of the filesystem on disk where a linear list of inodes is kept. The in-core inode table stores additional information so that the kernel accesses the correct device if more than one filesystem exists.
Converting pathnames to inode numbers is a time-consuming process. It may require several disk accesses to read the inodes corresponding to the components of a directory pathname. The namei cache is used to reduce the number of times the disk must be accessed to find a file. Provided that a name component is less than 14 characters long, its name, inode number, and its parent inode number are placed in the namei cache located in memory. Pathname components longer than 14 characters are never cached. When a command wishes to open a file, the kernel first looks in the namei cache for each pathname component in turn. If it cannot find a component there, it retrieves the directory information from disk into the buffer cache and adds the entry to the namei cache if possible.