Chapter 10: File System...

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Chapter 10: File System Interface

Operating System Concepts

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Chapter 10: File-System Interface

File Concept Access Methods Directory Structure File System Mounting File Sharing Protection

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File Attributes

Name – only information kept in human-readable form.

Identifier – unique tag/number, identifies the file in the file system.

Type – needed for systems that support different types.

Location – pointer to file location on device. Size – current file size.

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File Attributes (Cont.)

Protection – controls who can do reading, writing, executing.

Time, date, and user identification – data for protection, security, and usage monitoring.

Information about files are kept in the directory structure, which is maintained on the disk.

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(a) fixed-size entries with the disk addresses and attributes (DOS) (b) each entry refers to an i-node. Directory entry contains

attributes. (Unix)

Implementing Directories structure

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Implementing Directories structure

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File Operations

Create Write Read Reposition within file – file seek Delete Truncate

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File Operations (Cont.)

Open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory.

Close (Fi) – move the content of entry Fi in memory to directory structure on disk.

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File Operations (Cont.)

File locking Advisory locking: requires cooperation from the

participating processes Mandatory locking: causes the kernel to check

every open, read, and write to verify that the calling process isn’t violating a lock on the given file

Operating System Concepts

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File Types – Name, Extension

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Access Methods Sequential Access access- read from the beginning, can’t skip around Corresponds to magnetic tape

Direct Access Random access- start where you want to start Came into play with disks

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Simulation of Sequential Access on a Direct-access File

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Directory Structure

disks are split into one or more partitions.

each partition contains information about files within it

The information is kept in entries in a device directory or volume table of contents

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A Typical File-system Organization

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Operations Performed on Directory

Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system

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Organize the Directory (Logically) to Obtain

Efficiency – locating a file quickly. Naming – convenient to users. Two users can have same name for different

files. The same file can have several different

names. Grouping – logical grouping of files by

properties, (e.g., all Java programs, all games, …)

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Single-Level Directory

A single directory for all users.

Naming problem Grouping problem

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Two-Level Directory Separate directory for each user.

•Path name •Can have the same file name for different user •Efficient searching •No grouping capability

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Tree-Structured Directories

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Tree-Structured Directories (Cont.)

Efficient searching

Grouping Capability

Current directory (working directory) cd /spell/mail/prog type list

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Absolute or relative path name

Creating a new file is done in current directory.

Delete a file rm <file-name>

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Tree-Structured Directories (Cont.)

Creating a new subdirectory is done in current directory. mkdir <dir-name>

Example: if in current directory /mail mkdir count

mail

prog copy prt exp count

Deleting “mail” ⇒ deleting the entire subtree rooted by “mail”.

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Acyclic-Graph Directories Have shared subdirectories and

files.

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Acyclic-Graph Directories (Cont.)

Two different names (aliasing)

If dict deletes count ⇒ dangling pointer. Solutions: Backpointers, so we can delete all pointers.

Entry-hold-count solution.

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A UNIX directory tree.

Operating System Concepts

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Example: Linux hard link and soft link

Explained following: https://www.ibm.com/developerworks/cn/linux/l-

cn-hardandsymb-links/

Operating System Concepts

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General Graph Directory

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General Graph Directory (Cont.)

How do we guarantee no cycles? Allow only links to file not subdirectories. Garbage collection. Every time a new link is added use a cycle

detection algorithm to determine whether it is OK.

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File System Mounting

A file system must be mounted before it can be accessed.

An unmounted file system (I.e. Fig. 11-11(b)) is mounted at a mount point.

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(a) Existing. (b) Unmounted Partition

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Mount Point

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Example: Mounting iso file under Linux

Operating System Concepts

mkdir -p /mnt/disk mount -o loop disk1.iso /mnt/disk umount /mnt/disk

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File Sharing Sharing of files on multi-user systems is

desirable. Sharing may be done through a protection

scheme. On distributed systems, files may be shared

across a network. Network File System (NFS) is a common

distributed file-sharing method.

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NFS usage example

NFS server: directory to be shared [root@c01n08 tmp]# vi /etc/exports /tmp *(rw,no_root_squash)

[root@c01n08 tmp]# service nfs restart

NFS client: access the shared directory [root@c01n09 ~]# mkdir tmp [root@c01n09 ~]# mount -t nfs c01n08:/tmp tmp

Operating System Concepts

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Protection File owner/creator should be able to control: what can be done by whom

Types of access Read Write Execute Append Delete List

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Access Lists and Groups Mode of access: read, write, execute Three classes of users RWX a) owner access 7 ⇒ 1 1 1

b) group access 6 ⇒ 1 1 0 c) public access 1 ⇒ 0 0 1 Ask manager to create a group (unique name),

say G, and add some users to the group. For a particular file (say game) or subdirectory,

define an appropriate access. owner group public

chmod 761 game Attach a group to a file chgrp G game

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A Sample UNIX Directory Listing

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Directory access permission

做实验证明： root$ mkdir test/d1 -p root$ touch test/f1 root$ echo "test" > test/f2

验证读权限： root$ chmod 004 test (r读权限) robin$ ls test (可以看到d1, f1, f2) robin$ cat test/f2 (Permission denied) root$ chmod 005 test (rx读执行权限) robin$ cat test/f2 （看到“test”了）

Operating System Concepts

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Directory access permission

验证执行权限： root$ chmod 001 test (执行权限) robin$ ls test (这边没有东西了哦！) robin$ cat test/f2（可以看到“test”）

验证写权限： root$ chmod 002 (w写权限） robin$ echo "test2" > test/f3 (Permission denied) root$ chmod 003 (wx写执行权限） robin$ echo "test2" > test/f3 (成功）

Operating System Concepts

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Windows XP Access-control List Management