1.7 I/o devices and file systems  (Page 7/9)

Unix directories

Unix approach compares the directory structure to a tree.

• Directories are stored on disk just like regular files (i.e. file descriptor with 13 pointers, etc.). User programs can read directories justlike any other file (try it!). Only special system programs may write directories.
• Each directory contains pairs. The file pointed to by the index may be another directory. Hence, get hierarchical tree structure, name with/usr/local.
• There is one special directory, called the root. This directory has no name, and is the file pointed to by descriptor 2 (descriptors 0 and 1have other special purposes).

It is very nice that directories and file descriptors are separate, and the directories are implemented just like files.This simplifies the implementation and management of the structure (can write "normal" programs to manipulate them as files).

Working directory: it is cumbersome constantly to have to specify the full path name for all files.

• In Unix, there is one directory per process, called the working directory, that the system remembers.
• When it gets a file name, it assumes that the file is in the working directory. "/" is an escape to allow full path names.
• Many systems allow more than one current directory. For example, check first in A, then in B, then in C. This set of directories is called thesearch path or search list. This is very convenient when working on large systems with many different programmers in different areas.
• For example, in Unix the shell will automatically check in several places for programs. However, this is built into the shell, not intoUnix, so if any other program wants to do the same, it has to rebuild the facilities from scratch.
• This is yet another example of locality.

Windows (nt) file system

Background

The Windows file system is called NTFS, and was introduced with Windows NT 4.0 and is the standard file system on Windows 2000and later systems, such as Windows XP. Its goal was to solve the size, performance, reliability, and flexibility limitations in the DOS (aka "FAT" filesystem).

It has a general similarity to the FAT file system in that all files are described in a single table, called the Master File Table(MFT). However, it has more modern characteristics in that all components are files, including:

The file system also has features to support redundancy and transactions, which we will not discuss. A great reference fordetails is the book: Inside the Windows NT File System by Helen Custer, published by (not surprisingly) Microsoft Press.

Disk layout

Disks are divided in fixed size regions:

• Each region is called a volume.
• Each volume can contain a different kind of file system, such as NTFS, FAT, or even Unix.
• Since each volume is a separate file system, it has its own root directory.
• Multiple volumes allow for fixed limits on the growth of a particular file tree, such as limiting the size of temporary file space.
• Multiple volumes also allow a single disk to contain multiple, separating bootable operating systems.