Annotated by B. Hirsbrunner File Systems Chapter 5 5.1 Files 5.2 Directories 5.3 File System Implementation 5.4 Security 5.5 Protection Mechanism 5.6 Overview

Annotated by B. Hirsbrunner

File Systems

Chapter 5

5.1 Files 5.2 Directories 5.3 File System Implementation5.4 Security5.5 Protection Mechanism5.6 Overview5.7 Implementation of the Minix 3 File System

Andrew S. Tanenbaum and Albert S. WoodhullOperating Systems (The Minix Book), 3rd editionPrentice Hall © 2006

Lecture 11, 4 December 2012

Annotated by B. Hirsbrunner 2

5. Essential Requirements for Long-term Information Storage

1. Must store large amounts of data

2. Information stored must survive the termination of the process using it

3. Multiple processes must be able to access the information concurrently


5.1.1 File Naming

Fig. 5.1 Typical file extensions.


5.1.2 File Structure

Fig. 5.2 Three kinds of filesa) byte sequenceb) record sequencec) tree


5.1.3 File Types

Fig. 5.3 (a) An executable file (b) An archive

Magic number: identifies the file as an executable


5.1.4 File Access• Sequential access

– read all bytes/records from the beginning– cannot jump around, could rewind or back up– convenient when medium was magnetic tape

• Random access– bytes/records read in any order– essential for data base systems– read can be implemented by :

• move file marker (seek), then read sequentially (Unix, Windows)

• Give the position in the file to start reading at


5.5.5 File Attributes

Fig. 5.4 Some possible file attributessee OS X packages and meta data !


5.1.6 File Operations

• Create

• Delete

• Open

• Close

• Read

• Write

• Append

• Seek

• Get attributes

• Set attributes

• Rename

• Lock

The most common system calls relating to files:


5.2.1 Simple Directories

Fig. 5.5a) A simple directory (e.g. Windows)

o containing fixed size entrieso with disk addresses and attributes in directory entry

b)Directory in which each entry just refers to an i-node (e.g. Unix)


5.2.2 Hierarchical Directory Systems

Fig 5.6.a A single level directory system– contains 4 files– owned by 3 different people, A, B, and C


5.2.2 Hierarchical Directory Systems

Fig. 5.6.b A hierarchical directory system


Fig. 5.7 A UNIX directory tree

5.2.3 Path Names


5.6.6 Directories and Paths: Minix3

Fig. 5.38 (a) Root file system. (b) An unmounted file system. (c) The result of mounting the file system of (b) on /usr/

Special file: contains no data, but provides a mechanism to map physical devices to file names

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5.2.4 Directory Operations

• Create

• Delete

• Opendir

• Closedir

• Readdir

• Rename

• Link

• Unlink

The most common system calls relating to directories:

hard / symbolic link, see later (variants: alias in OS X, shortcut in Windows)


5.3.1 File System Layout

Fig. 5.8 A possible file system layoutMBR = Master Boot Record

Super block: contains all the key parameters about the file sytem


5.6.2 File System Layout: MINIX 3

Fig. 5.34 Disk layout for a floppy disk or small hard disk partition, with 64 i-nodes and a 1-KB block size (i.e., two consecutive 512-byte sectors are treated as a single block).

zone: 1, 2, 4, …, 2n blocks 16


5.3.2 Contiguous File Allocation

Idea: allocate n consecutive blocks on the disk

Advantage•Very simple to implement•Excellent read performance

Major drawback•in time, the disk becomes fragmented•often, the final file size is not known at creation time

Usage•magnetic disk file systems•optical write-once media: CD-ROMs, DVDs, …


5.3.2 Linked List File Allocation

Fig. 5.9 Storing a file as a linkedlist of disk blocks

Fig 5.10 Linked list allocation using a file allocation table in RAM


5.3.2 i-nodes

Fig. 5.11 An i-node with three levels of indirect blocks


5.3.3 Shared Files

Fig. 5.12 File system containing a shared file

Hard link vs symbolic (or soft) link

Unix command: link


5.3.3 Shared Files : hard link

Fig. 4.17 a) Situation prior to linkingb) After the hard link is createdc) After the original owner removes the file

Andrew S. Tanenbaum: "Modern Operating Systems", 3rd edition, Prentice Hall © 2009


Example: rsync to mimic Apple’s TimeMachine

Source: http://blog.interlinked.org/tutorials/rsync_time_machine.html

Question: hard link or soft link ?


Operations required to remove a file in UNIX• Remove the file from its directory.

• Release the i-node to the pool of free i-nodes.

• Return all the disk blocks to the pool of free disk blocks.

4.3.6 Journaling File Systems

Problem if the system crashes during these actions

SolutionWrite a log entry listing the three actions to be completed and write it to disk + atomic transaction (i.e. 'begin transaction', 'end transaction')

Andrew S. Tanenbaum: "Modern Operating Systems", 3rd edition, Prentice Hall © 2009 23


Fig. 4.18 Position of the virtual file system.

4.3.7 Virtual File Systems

Andrew S. Tanenbaum: "Modern Operating Systems", 3rd edition, Prentice Hall © 2009

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5.3.3 Directories in Windows 98

Fig. 5.14 An entry for (part of) a long file name in Windows 98.

Fig 5.13 A Windows 98 base directory entry.

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5.3.3 Directories in Unix

Fig. 5.15 A Version 7 UNIX directory entry


5.3.3 Directories in Unix

Fig. 5.16 The steps in looking up /usr/ast/mbox


5.3.6 Reducing Disk Arm Motion

Fig. 5.21

a)i-nodes placed at the start of the disk.

b)Disk divided into cylinder groups, each with its own blocks and i-nodes.

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Annotated by B. Hirsbrunner File Systems Chapter 5 5.1 Files 5.2 Directories 5.3 File System Implementation 5.4 Security 5.5 Protection Mechanism 5.6 Overview