1. Operating System
File System
Monday, August 11, 2008

2. Objectives Understand basic concepts of file and file system
Understand physical and logic structure
Know the way for managint file store space
Know the file sharing and protection
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3. Roadmap File Concept Access Methods Directory Structure
File System Implementation
Allocation Methods
Free-Space Management
File Sharing and Protection
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4. File Concept
A file is a named collection of related information that recorded on secondary storage.
From a user’s perspective, a file is the smallest allotment storage.
The file is an entry in a directory.
The file may have attributes (name, creator, date, type, permissions)
The file may have structure ( O.S. may or may not know about this.) It's a tradeoff of power versus overhead.
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5. File Structure
Many different types of information may be stored in a file, according to which a file has a certain defined structure
Text file: a sequence of characters organized into lines.
Source file: a sequence of subroutines and functions, each of which is further organized as declarations followed by executable statements.
Object file: a sequence of bytes organized into blocks understandable by the system’s linker.
Executable file: a series of code sections that the loader can bring into memory and execute.
文件根据其类型具有一定结构。
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6. File Attributes A file typically consist of:
Name: only information kept in human-readable form
Identifier: unique tag, identifies the file
Type: needed for systems that support different types
Location: pointer to file location on device
Size: current file size
Protection: controls who can do reading, writing, executing
Time, date, and user identification: data for protection, security, and usage monitoring
文件是有名称的，以方便用户通过名称对之加以引用。有了名称之后，文件就独立于进程、用户、甚至创建它的操作系统了。
8/11/2008 6:14 PM

7. File Attributes A file typically consist of:
Name: only information kept in human-readable form
Identifier: unique tag, identifies the file
Type: needed for systems that support different types
Location: pointer to file location on device
Size: current file size
Protection: controls who can do reading, writing, executing
Time, date, and user identification: data for protection, security, and usage monitoring
文件是有名称的，以方便用户通过名称对之加以引用。有了名称之后，文件就独立于进程、用户、甚至创建它的操作系统了。
Information about files are kept in the directory structure, which is maintained on the disk.
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8. File Operations
A file is an abstract data type. To define a file properly, we need to consider the operations that can be performed on files.
Create: find space and make an entry for the file
Write: keep and update the write pointer
Read: keep and update the read pointer
File seek: reposition within file
Delete: release space and erase the directory entry
Truncate: allows all attributes to remain unchanged except for file length.
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9. 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
绝大多数文件操作都涉及到为给定文件搜索相关目录条目。为了避免这种不断搜索操作，许多系统要求在首次使用文件时，使用系统调用open。操作系统维护一个包含所有打开文件的信息的表（打开文件表）。当需要一个文件操作时，通过该表的一个索引指定文件，而不需要搜索。当不再使用时，进程可以关闭文件，操作系统从打开文件表中删除这一条目。
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10. Open Files Several pieces of data are needed to manage open files:
File pointer: pointer to last read/write location, per process that has the file open
File-open count: counter of number of times a file is open – to allow removal of data from open-file table when last processes closes it
Disk location of the file: cache of data access information
Access rights: per-process access mode information
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11. Open File Locking Provided by some operating systems and file systems
Mediates access to a file
Mandatory or advisory:
Mandatory – access is denied depending on locks held and requested
Advisory – processes can find status of locks and decide what to do
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12. File Locking Example – Java API
import java.io.*;
import java.nio.channels.*;
public class LockingExample {
public static final boolean EXCLUSIVE = false;
public static final boolean SHARED = true;
public static void main(String arsg[]) throws IOException {
FileLock sharedLock = null;
FileLock exclusiveLock = null;
try {
RandomAccessFile raf = new RandomAccessFile("file.txt", "rw");
// get the channel for the file
FileChannel ch = raf.getChannel();
// this locks the first half of the file - exclusive
exclusiveLock = ch.lock(0, raf.length()/2, EXCLUSIVE);
/** Now modify the data */
// release the lock
exclusiveLock.release();
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13. File Locking Example – Java API
// this locks the second half of the file - shared
sharedLock = ch.lock(raf.length()/2+1, raf.length(), SHARED);
/** Now read the data */
// release the lock
exclusiveLock.release();
} catch (java.io.IOException ioe) {
System.err.println(ioe);
}finally {
if (exclusiveLock != null)
if (sharedLock != null)
sharedLock.release(); }
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14. File Types – Name, Extension
当设计文件系统时，总是要考虑操作系统是否应该识别和支持文件类型。如果能识别文件类型，就能按合理方式对文件进行操作。
实现文件类型通常的技术是在文件名称内包含类型。名称可分为两部分：名称和扩展名。
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15. Roadmap File Concept Access Methods Directory Structure
File System Implementation
Allocation Methods
Free-Space Management
File Sharing and Protection
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16. Sequential Access
Sequential Access: information in the file is processed in order, one record after another
Implemented by the filesystem.
Data is accessed one record right after the last.
Reads cause a pointer to be moved ahead by one.
Writes allocate space for the record and move the pointer to the new End Of File.
Such a method is reasonable for tape
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17. Direct Access
Direct Access: a file is made up of fixed-length logical records that allow programs to read and write records rapidly in no particular order.
Method useful for disks.
The file is viewed as a numbered sequence of blocks or records.
There are no restrictions on which blocks are read/written in any order.
User now says "read n" rather than "read next".
"n" is a number relative to the beginning of file, not relative to an absolute physical disk location.
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18. Simulation of Sequential Access on a Direct-access File
可以用直接访问来模拟顺序访问，而用顺序访问模拟直接访问则是极为低效和笨重的。
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19. Other Methods
Other access methods can be built on top of a direct-access method.
Indexed
These methods generally involve the construction of an index for the file.
The index contains pointers to the various blocks.
An index block says what's in each remaining block or contains pointers to blocks containing particular items. Suppose a file contains many blocks of data arranged by name alphabetically.
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20. Examples
Example 1: Index contains the name appearing as the first record in each block. There are as many index entries as there are blocks.
Example 2: Index contains the block number where "A" begins, where "B" begins, etc. Here there are only 26 index entries.
Adams
Arthur
Asher
Smith, John | data
Smith
Adams
Adams | Data
Baker
Arthur | Data
Charles
Asher | Data
Baker | Data
Saarnin
Saarnin | data
Smith, John | data
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21. Roadmap File Concept Access Methods Directory Structure
File System Implementation
Allocation Methods
Free-Space Management
File Sharing and Protection
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22. Directory Structure Directories maintain information about files:
Name The user visible name.
Type The file is a directory, a program image, a user file, a link, etc.
Location Device and location on the device where the file header is located.
Size Number of bytes/words/blocks in the file.
Position Current next-read/next-write pointers.
Protection Access control on read/write/ execute/delete.
Usage Open count
Usage time of creation/access, etc.
Mounting a filesystem occurs when the root of one filesystem is "grafted" into the existing tree of another filesystem.
There is a need to PROTECT files and directories.
Actions that might be protected include:
read, write, execute, append, delete, list
In Memory only!
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23. Directory Structure
A collection of nodes containing information about all files
F 1
F 2
F 3
F 4
F n
Directory
Files
Both the directory structure and the files reside on disk
Backups of these two structures are kept on tapes
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24. A Typical File-system Organization
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25. Information in a Device Directory
Name
Type
Address
Current length
Maximum length
Date last accessed (for archival)
Date last updated (for dump)
Owner ID
Protection information (discuss later)
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26. 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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27. Organize the Directory 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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28. Logical Structure of A Directory
The most common schemes for defining logical structure of a directory are:
Single-Level Directory
Two-Level Directory
Tree-Structured Directory
Acyclic-Graph Directory
General Graph Directory
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29. Single-Level Directory
A single directory for all users
Easy to support and understand
It has significant limitations, since all files must have unique names in the same directory.
易于支持和理解，但是由于文件名需要唯一造成很复杂的情况
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30. Two-Level Directory Separate directory for each user
When a user refers to a particular file, only his own UFD is searched.
Different users may have files with the same name.
Difficult to cooperate on some task and to access one another’s files between users.
单层目录通常会在不同用户之间引起文件名称的混淆。标准解决方法是为每个用户创建独立目录。
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31. Tree-Structured Directories
The most common directory structure.
The tree has a root directory
Every file in the system has a unique path name
A path name is the path from the root, through all the
subdirectories,
to specified file.
树是最为常用的目录结构。树有根目录。系统内的每个文件都有惟一路径名。路径名是从根经过所有子目录再到指定文件的路径。
8/11/2008 6:14 PM

32. Tree-Structured Directories
The most common directory structure.
The tree has a root directory
Every file in the system has a unique path name
A path name is the path from the root, through all the subdirectories, to specified file.
树是最为常用的目录结构。树有根目录。系统内的每个文件都有惟一路径名。路径名是从根经过所有子目录再到指定文件的路径。
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33. Acyclic-Graph Directories
Have shared subdirectories and files
树形结构禁止共享文件和目录。无环图允许目录含有共享子目录和文件。
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34. General Graph Directory
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35. 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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36. Keystone Three access methods Common directory structure
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37. Operating System
File and Directory
Monday, August 11, 2008