Directories and Links

Optional readings for this topic from Operating Systems: Principles and Practice: Sections 13.1-13.2.

Naming: how do users refer to their files? How does OS find file, given name?

First step: inode has to be stored on disk, so it will persist across system reboots.

Early UNIX versions: all inodes stored in a fixed-size array on disk.

Originally: entire inode array was at the outer edge of the disk. Result: long seeks between inodes and file data. First improvement: place inode array mid-way across disk. Second improvement: many small inode arrays spread across disk, so inodes can be near to file data.

Space for inodes is fixed when the disk is initialized, and can't be changed.

UNIX/Linux terms:

Index of inode in the inode array: i-number. Internally the OS uses the i-number as an identifier for the file.

When a file is open, its inode is kept in main memory. When the file is closed, the inode is written back to disk.

File naming: users want to use text names to refer to files. Special disk structures called directories are used to map names to i-numbers.

Early approaches to directory management:

A single directory for the entire disk:
- Many early personal computers worked this way.
A single directory for each user (e.g. TOPS-10):
- Avoids problems between users, but still makes it hard to organize information.

Modern systems support hierarchical directory structures. UNIX/Linux approach:

Directories are stored on disk just like regular files (i.e. inode with 14 pointers, etc.) except inode has special "type" bit set to indicate that it's a directory.
A simple approach: each directory contains <name, i-number> pairs in no particular order.
The file referred to by the i-number may be another directory. Hence, get hierarchical tree structure. Names have slashes separating the levels of the tree.
There is one special directory, called the root. This directory has no name; it has i-number 1 (i-number 0 is not used).
Only the operating system can read and write directories.

Working directories

Cumbersome to specify the full path name for all files.

OS remembers the i-number for one distinguished directory per process, called the working directory.

If a file name doesn't start with "/" then it is looked up starting in the working directory.

Names starting with "/" are looked up starting in the root directory.