General Objective:To understand the file system management. Specific Objectives: At the end of the unit you should be able to:

Transcription

1 F2007/Unit8/1 Unit 8 OBJECTIVES General Objective:To understand the file system management Specific Objectives: At the end of the unit you should be able to: define the file system management list the objective of file system management describe the concept and design of file directory, sharing and file protection describe the secondary storage organization program

2 F2007/Unit8/2 INPUT 8.0 Introduction In this unit, we examine those basic elements. We begin with an overview of files and file management systems. This is followed by a look at alternative organizations or files. Although file organization is generally beyond the scope of the operating system, it is essential to have a general understanding of the alternatives in order to appreciate some of the design trade offs involved in file management. 8.1 File system management objectives A file management system is that set of system software that provides services to users and applications related to the use of files. Typically, the only way that a user or application may access files is through the file management system. This relieves the user or programmer of the necessity of developing special purpose software for each application and provide the system with a means of controlling its most important asset.[gros86] suggests the following objectives for file management system: To meet the data-management needs and requirements of the user, which include storage of data and the ability to perform the operations listed earlier To guarantee, the extent possible that the data in the file are valid

3 F2007/Unit8/3 To optimize performance both from the system point of view in terms of overall throughput and from the user s point of view in terms of response time To provide I/O support for a variety of types of storage device To minimize or eliminate the potential for lost or destroyed data To provide a standardized set of I/O interface routines To provide I/O support for multiple users in the case of multiple user systems 8.2 Concept and design Computer can store information on several different storage media such as magnetic disks, magnetic tapes and optical disks. So that the computer systems will be convenient to use, the operating system provides a uniform logical view of information storage. The operating system abstract from the physical properties of its storage devices to define a logical storage unit, the file. File are mapped, by the operating system, on to physical devices. This storage device are usually non volatile, so the contents are persistent thru power failures and systems reboots File directory Associated with any file-management system and collection of files is a file directory. The directory contains information about the files including attributes, location and ownership. Much of this information, especially that concern with storage, is managed by the operating system. The directory is itself a file, owned by the operating system and accessible by various to users and applications, information is generally provided indirectly by system routines. Thus, users

4 F2007/Unit8/4 cannot directly access the directory even in read-only mode. We can describe the most common schemes for defining the logical structure of a directory: 1. Single level Directory The simplest directory structure is the single-level directory. All files are contained in the same directory, which is easy to support and understand. A single level directory has significant limitation, however, when the number of the file increases or when there is more than one user. Since all files are in the same directory, they must have unique names. If we have two users who call their data file test, then the unique name rule is violated. For example in one programming class 23 student call the program for their second assignment prog2: another 11 call it assign2. Although file names are generally selected to reflect the content of the file, there are often limited in length. The MS-DOS operating system allows only 11 characters file names: Unix allows 255 characters. Even with a single user, as the number of files increases, it becomes difficult to remember the name of all the file, so as to create only file with unique name. It is not uncommon for a user to have 100 of files on one computer system and an equal number of additional files on another system. In such an environment, keeping track of so many files is a daunting task. Directory Cat Bo A Test Data Mail Cont hex records Figure 8.1 Single level directory (Source: Galvin, Silverchatz (1998) Operating System Concepts)

5 F2007/Unit8/5 2. Two level directory The major disadvantage to a single level directory is the confusion of files names between different users. The standard solution is to create a separate directory for each user. In the two level directory structures, each user has her own user file directory (UFD). Each UFD has a similar structure, but lists only the files of a single user. When a user job starts or a user log in, the system master file directory (MFD), is search. The master file directory is index by user name or account number, and each entry point to the UFD for that user. When a user refers to a particular file, only his own UFD is search. Thus different users may have files with the same name, as long as all the file names within it UFD are unique. Master file directory User 1 User 2 User 3 User 4 Cat Bo a test a data a test x data a Figure 8.2 Two Level directory (Source: Galvin, Silverchatz (1998) Operating System Concepts) 3. Multilevel directory Once we have seen how to view a two level directory as two-level tree, the natural generalization is to extend the directory structure to a tree of a arbitrary

6 F2007/Unit8/6 height. This generalization allows users to create their own subdirectories and to organize their files accordingly. The MS-DOS system for instance is structured as a tree. In fact a tree is the most common directory structure. The tree has a root directory. Every files in the system has a unique path name. A path name is the path from the root through all the sub directories to a specified file. spell bin program stat mail dist find count hex reorder p e mail prog copy prt exp reorder list find hex count list obj spell all last first Figure 8.3 Tree structured directory (Source: Galvin, Silverchatz (1998) Operating System Concepts) File Sharing In a multi user system, there is almost always a requirement for allowing files to be shared among a number of users. Two issues arise: access rights and the management of simultaneous access.

7 F2007/Unit8/7 Access Rights The file systems provide a flexible tool for the following extensive file sharing among users. The file system should provide a number of options so that the way in which a particular file is accessed can be controlled. Typically, users or groups of users are granted certain access rights to a file. A wide range of access rights has been used. The following list is representative of access rights that can be assigned to a particular user for a particular file: None: The user may not even learn of the existence of the file much less access it. To enforce this restriction, the user is not allowed to read the user directory that includes this file. Knowledge: The user can determine that the file exists and who its owner is. The user is then able to petition the owner for additional access rights Execution: The user can load and execute a program but cannot copy it. Proprietary programs are often made accessible with this restriction. Reading: The user can read the file for any purpose, including copying and execution. Some system is able to enforce a distinction between viewing and copying. In the former case, the content of the file can be displayed to the user, but the user has no means for making copy. Appending: The user can add data to the file, often only at the end, but cannot modify or delete any of the file s content. This right is useful in collecting data from a number of sources. Updating: The user can modify, delete and add to the file s data. Updating normally includes writing the file initially, rewriting it completely or in part and removing all or portion of the data. Some systems distinguish among different degrees of updating.

8 F2007/Unit8/8 Changing protection: The user can change the access rights granted to other users. Typically this right is held only by the owner of the file. In some systems, the owner can extend this right to others. To prevent abuse of this mechanism, the file owner is typically able to specify which rights can be changed by the holder of this right. Deletion: The user can delete the file from the file system. This right can be considered to constitute a hierarchy with each right implying those that precede it. Thus, if a particular user is granted the updating right for a particular file, then that user is also granted the following rights: Knowledge, execution, reading and appending.

9 F2007/Unit8/9 ACTIVITY 8A TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE THE NEXT INPUT...! 8.1 Give me 3 objectives of file management system 8.2 What is the major disadvantage to a single directory?

10 F2007/Unit8/10 FEEDBACK TO ACTIVITY 8A 8.1 To meet the data-management needs and requirements of the user, which include storage of data and the ability to perform the operations listed earlier To guarantee, the extent possible that the data in the file are valid To optimize performance both from the system point of view in terms of overall throughput and from the user s point of view in terms of response time To provide I/O support for a variety of types of storage device To minimize or eliminate the potential for lost or destroyed data To provide a standardized set of I/O interface routines To provide I/O support for multiple users in the case of multiple user systems 8.2 The major disadvantage to a single level directory is the confusion of files names between different users.

11 F2007/Unit8/11 INPUT 8.3 Secondary storage organization program A file may contain more than one block of secondary storage. It means few block should linking to form file. There are three methods which are: Linking block File map Index block Linking block Every block that use to from a file will link using a pointer. The pointer in UFD (User file directory) link to the first block in a chain. The Problem to this method is there are many access to disk and need to be done to find the last file, for example to delete file, clear information about the earlier position of the file. File block Figure 8.4: Linking Block File map UFD This method End block linking the block to form a file and were recorded using file map. It is known as file scheduling. The pointer in the file of UFD will

12 F2007/Unit8/12 link to the location in the map file which is representing the first block to form the file. The last block will stated using zero pointers UFD File map Figure 8.5: File map Index Block

13 F2007/Unit8/13 This method will link the entire block using the index block. The pointer in the UFD will link to that index block. The advantage of this method is file can be accessed without parallel system. While the disadvantages are too many space should be used to keep the index block. UFD 0 Index Block File Block Figure 8.6: Index Block ACTIVITY 8B

14 F2007/Unit8/ A file may contain more than one block of secondary storage. Its mean few block should link to form file. There are 3 methods which are: i. i i g B o k ii. F l e M iii. In x l k 8.4 Complete the file mapping diagram with the given word UFD File map FEEDBACK TO ACTIVITY 8B

15 F2007/Unit8/ i. Linking Block ii. File Map iii. Index Block UFD File Map SELF-ASSESSMENT 1

16 F2007/Unit8/16 Question 8-1 a. What are the 3 level of directory? i. ii. iii. b. What are the differences between each level? SELF-ASSESSMENT 2

17 F2007/Unit8/17 Question 8-2 a. File mapping is also known as file. b. Explain the index block method by drawing the diagram. FEEDBACK TO SELF ASSESSMENT 1 Question 8-1

18 F2007/Unit8/18 a. i. Single Level ii. Two Level iii. Multi Level b.please refer the input given and discuss with your lecturer. FEEDBACK TO SELF ASSESSMENT 2 Question 8-2

19 F2007/Unit8/19 a. File mapping is also known as file scheduling. b. Please refer the input given and discuss with your lecturer.