File System Interface: Overview. Objective. File Concept UNIT-IV FILE SYSTEMS

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1 UNIT-IV FILE SYSTEMS File System Interface: File Concept Access Methods Directory Structure File System Mounting Protection Overview For most users, the file system is the most visible aspect of an operating system. It provides the mechanism for on-line storage of and access to both data and programs of the operating system and all the users of the computer system. the file system consists of two distinct parts; a collection of files, each storing related to data, and a directory structure, which organizes and provides information about all the files in the system. In this chapter, we consider the various aspects of files and the major directory structures. we also discuss the ways to handle file protection, necessary when multiple users have access to files and we want to control by whom and in what ways the files are accessed. Objective To explain the function of file systems To describe the interfaces to file systems To discuss file-system design tradeoffs, including access methods, file sharing, file locking, and directory structures To explore file-system protection File Concept A file is a collection of similar records. The file is treated as a single entity by users and applications and may be referred by name. Files have unique file names and may be created and deleted. Restrictions on access control usually apply at the file level. A file is a container for a collection of information.

2 The file manager provides a protection mechanism to allow machine users to administer how processes executing on behalf of different users can access the information in a file. File protection is a fundamental property of files because it allows different people to store their information on a shared computer. File represents programs and data. Data files may be numeric, alphabetic, binary or alphabetic, binary or alpha numeric. Files may be free form, such as text files. In general, file is sequence of bits, bytes, lines of records. A File has a certain defined structure according to its type. 1. Text file 2. Source file 3. Executable file 4. Object file A text file is a sequence of characters organized into lines. A source file is a sequence of subroutine and functions. An object file is a sequence of bytes organized into blocks understandable by the systems linker. An executable file is a series of code sections that the loader can bring into memory and execute. File Attributes File attributes vary from one operating system to another. The common file attributes are 1. Name 2. Identifier 3. Type 4. Location 5. Size 6. Protection 7. Time, date and user identification o The symbolic file name is the only information kept in human readable form. o Identifier is the unique tag which identifies the file within the file system. It is usually a number.

3 o Some systems support different types of file. The file type information is required in this system. o Location information is a pointer to a device and to the location of the file on that device. o Size attributer is one of the important factors. Size attributes is the fundamental property of the file. o Access control information determines who can do reading, writing, executing and son on. o Time, date and user identification may be kept for creation, last modification and last use. These data can be useful for protection, security and usage monitoring. All information about files is kept in the directory structure. It is stored on the secondary storage device. File Operations A file is an abstract data type. Operation on file, operating system provides system calls for creating, deleting, read etc. Basic operations on files are 1. Create a file 2. Writing a file 3. Reading a file 4. Delete a file 5. Truncating 6. Repositioning within a file. Create a file: For creating a file, address space in the file system is required. After creating a file, entry of the file is made in the directory. The directory entry records the name of the file and the location in the file system.

4 Writing a file: System call is used for writing into file. It is required to specify the name of the file and information to be written to the file. According to the file name, system will search the name in the directory to find the location of the file. Reading a file: To read a file, system call is used. It requires the name of file and memory address. Again the directory is searched for the associated directory entry and the system needs to keep a read pointer to the location in the file where the next read is to take place. Delete a file: System will search the directory with which file to be deleted. If directory entry is found, it releases all file space. That free space can be reused by another (user) files. Truncating a file: User may want to erase contents of file but keep its attributes. Rather than forcing the user to delete a file and then recreate it, truncation function allows all attributes to remain unchanged except for file length. Repositioning within a file: The directory is searched for the appropriate entry, and the current file position is set to a given value. Repositioning within a file does not need to involve any actual I / O. This file operation is also known as file seeks. Several pieces of information are associated with open file. They are 1. File pointer 2. File open count 3. Disk location of the file 4. Access rights

5 File pointer: This pointer is unique to each process operating on the file, and therefore must be kept separate from the on-disk file attributes. File open count: As files are closed, the operating system must reuse its open-file table entries, or it could run out of space in the table. This counter tracks the number of opens and closes and reaches zero on the last close. The system can then remove the entry. Disk location of the file Most file operations require the system to modify data within the file. This information needed to locate the file on disk is kept in memory to avoid having to read it from disk for each operation. Access rights Each process opens a file in an access mode. This information is stored on the perprocess table so the operating system can allow or deny subsequent I/O request. File Types A common technique for implementing file types is to include the type as part of the file name. The name is split into two parts: a name and an extension, following table gives the file type with usual extension and function.

6 Access Methods Files store information. When it is used, this information must be accessed and read into computer The information in the file can be accessed in several ways. Different types of file access methods are 1. Sequential access 2. Direct access Sequential Access: Sequential access is the simplest method. Information in the file is sequentially accessed. (I.e. one record of other record). Editors and compilers usually access files in this fashion. Normally read and write operations are done on the files. A read operation reads the next portion of the file and automatically advances a file pointer, which tracks I / O location, write operation appends the end of file and such file can be reset to the beginning. Figure show the sequential access model.

7 Sequential-access file Direct Access: Direct access allows random access to any file block. This method is based on a disk model of file. A file is made up of fixed length logical records. It allows programs to read and write records rapidly in no particular order. A direct-access file allows arbitrary blocks to be read or written. For example, user may read block 13, then read block 99, then write block 12. In a direct access file, no restrictions for reading or writing a file in any sequence. For searching the record in large amount of information with immediate result, direct access method is suitable. Databases are often of this type. Not all operating system support both sequential and direct access for files some operating systems use sequential access and some operating system use direct access. It is easy to simulate sequential access on a direct access file. On the other hand, it is extremely inefficient and clumsy to simulate a direct access file on a sequential access file. Other access methods: Other access methods can be built on top of the direct access method. These methods generally involve the construction of an index for the file. The index like the index in the back of the book contains pointers to the various blocks. To find the record in the file, we first search the index, and then use the pointer to access the file directly and to find the desired record. To achieve the flexibility a structure is needed that employs multiple indexes.

8 In general indexed file, the concept of sequentially and a single key are abandoned. Records are only access through their indexes. Records of variable lengths can be used in this access. Two types of indexes are used. An exhaustive index contains one entry for every record n the main file. An index is itself organized as a sequential file for ease of searching. A partial index contains entries to records where the filed of interest exists. With records of variable length, some records will not contain all fields. When a new record is added to the main file, all the index files must be updated. Figure shows the index with relative files. Example of index and relative files Indexed files are used mostly in applications where timelines of information are critical and where data are rarely processed exhaustively. Examples are airline reservation systems and inventory control systems. Directory Structure Directories are basically symbol tables of file s a single fat directory can contain a list of all files in a system. A directory contains information about the files, including attributes, location and ownership. Operating system is manages this information. The directory is itself a filial owned by the operating system and accessible by

9 various file management routines. Thus user can not directly access the directory, even in read only mode. A typical file-system organization The simplest form of structure for a directory is that of a list of entries, on e for each file. Directory entries are added by the creation of files and of aliases to existing files. Entries are removed from the directories when files are deleted. Directories are frequently searched to locate and access files for processing and to add or to remove entries. To understand the requirements for a file structure, it is well to consider the type of operations that may be performed o the directory. 1. Search 2. Create a file 3. Delete file 4. Rename a file 5. List directory

10 Search: Directory structure is searched for finding particular file in the directory. Files have symbolic names and similar names may indicate a relationship between files. Create a file: When a new file is created, an entry must be added to the directory. Delete a file: When file is deleted, an entry must be removed from the directory. Rename a file: Name of the files must be changeable when the content or used of the file changes. Renaming a file ay also allow its position within the directory structure to be changed. List directory: All or protio9n of the directory may be requested. Request is made bad a sure and results in a listing of all files owned by that user plus some of the attributes of each file. Different types of directory structures are given below: 1. 1.Single level directory 2. Two level directory 3. Tree structured directory 4. Acyclic graph directories 5. General graph directory Single level Directory Single level directory structure is simple directory structure. All files are contained in the same directory. Figure shows single level directory structure. Single level directory is easy to implement and maintain.

11 Single level Directory Disadvantages 1) Not suitable for a large number of files and more than one user. 2) Because of single directory, files require unique file name. 3) It is difficult to remember the names of all the files as the number of files increases. Example: MS-DOS Operating system allows only 11 character file names whereas Unix allows 25 characters. Two-Level Directory In two level directories, each user has his own directory. It is called user file directory (UFD). Each user file directory has a similar structure. Figure show the two level directories. When a user refers to a particular file, only his own UFD is searched. Different users may have files with the same name, as long as the entire file name within each UFD are unique. Two-Level Directory structure

12 To create a file for a user, the operating system searches only that users directory to ascertain whether another file of that name exists. To delete a file, the operating system confines the search to the local UFD. Operating system can not accidentally delete another user s file that has the same name. Tree Structured Directories This allows the user to extend the directory structure to a tree of arbitrary height. This generalization allows the user to create their own sub directories and organize their files accordingly. MS-DOS system is a tree structure directory. It allows users to create their own subdirectory and to organize their files accordingly. A subdirectory contains a set of files or subdirectories. A directory is simply another file, but it is treated n a special way. All the directories have the same internal format. One bit in each directory entry defines the entry as a file (0) or as a subdirectory (1). Special system calls are used to create and delete directories. In normal use, each user has a current directory. Current directory should contain most of the files that are of current interest to the user. When a reference is made to a file, the current directory is searched. Path name is used to search or for any operation on file with another directory. Path names can be of two types: 1. Absolute path name 2. Relative path name Absolute path name begins at the root and follows a path down to the specified file, giving the directory names on the path. Relative Path name defines a path from the current directory. MS-DOS will not delete a directory unless it is empty. For deleting a directory, two approaches can be taken. 1. User must delete all the files from the directory. Make it empty directory. 2. In UNIX, rm command is used with some option for deleting directory. When a request is made to delete a directory, all that directory files and subdirectories are also to be deleted. A path to a file in a tree structured directory can be longer than that in a two level directory.

13 Advantages: 1. It allows users to create their own subdirectory. 2. User can access the files of other users. 3. It allows users to define their own search paths. Disadvantages: 1. Special system calls are required to create & delete directories. 2. It prohibits the sharing of files & directories. 3. Path to the file is longer than the two level directories. Tree Structured Directories Acyclic Graph Directories It allows directories to have shared subdirectories and files. But tree structured directories prohibits the sharing of files or directories. Same file air directory may be in two different directories. Graph with no cycles is a generalization of the tree structured subdirectory scheme. Shared files and subdirectories can be implemented by using links. A link is effectively a pointer to another file or subdirectory. A link is implemented as a absolute or relative path name. An acyclic graph directory structure is more flexible than is a simple tree structure, but sometimes it is more complex.

14 Acyclic Graph Directories structure Advantage: 1. An acyclic graph allows directories to have shared subdirectories and files. 2. Structure is more flexible than a simple tree structure. 3. Simple for traverse a file. Disadvantage: 1. Deletion of file is difficult. 2. A major problem with duplicate directory entries is maintaining consistency if the file is modified. General graph directory One serious problem with the acyclic-graph directory is ensuring that there are no cycles. If we start with a two-level directory and allow the users to create subdirectories, a tree structured directory results. It should fairly easy to see that simply adding new files and subdirectories to an existing tree structured directory preserves the treestructured nature. How ever, when we add links to an existing tree-structured directory, the tree structure is destroyed, resulting in a simple graph structure as shown in figure.

15 General graph directory The primary advantage is that the relative simplicity of the algorithms to traverse the graph and to determine when there are no more references to a file. We want to avoid traversing shared sections of an acyclic graph twice, mainly for performance reasons. If cycles are allowed to exist in the directory, we likewise want to avoid searching any component twice, for reasons of correctness as well as performance. A poorly designed algorithm might result in an infinite loop continually searching through the cycle and never terminating. One solution is arbitrarily to limit the number of directories that we will be accessed during a search. Garbage collection is necessary only because of possible cycles in the graph. File System Mounting File system must be mounted before it can be available to processes on the system. The mount procedure is straight forward. The operating system is given the name of the device, and the location with in the file structure at which to attach the file-system (or mount point). Typically a mount point is an empty directory at which the mounted file system ill be attached.

16 Procedure for mounting file system is as follows. 1. Mount point is an empty directory at which the mounted file system will be attached. 2. Name of the device and location within the file structure at which to attach the file system is required. 3. Operating system verifies that the device contains a valid file system. 4. Device driver is used by operating system for these verifications 5. Finally operating system mounts the file system at a specified mount point. Figure shows the file system with unmounted partition. The triangle represents sub tree of directories that are of interest. File system (a) Existing (b) Unmounted partition In above figure (a) an existing file system is shown and in figure (b) a unmounted partition residing on / device/disk is shown. At this point, only the fields on the existing file system can be accessed. Figure shows the effects of the mounting of the partition residing on /device/disk over user. If the partition is unmounted, the file system is restored to the situation shown in Figure above

17 Mount point Protection Information must be protected from a physical damage and improper access. I.e. reliability and protection. Type of Access Protection mechanisms provide controlled access by limiting the type of file access that can b made. Access is permitted or denied depending on several factories, of which I the type of access required. Different types of operations may be controlled in access type. These are 1. Read-read from the file 2. Write-write the file 3. Execute-load the file into memory and execute it 4. Append- write a new information at the end of the file 5. Delete- Delete the file and free its space for possible reuse 6. List- List the name and attributes of the file

18 Access Control 1. Various users may need different types of access to a file or directory. When a user request a particular file, the operating system checks the access list associated with that file. 2. If that user is listed for the requested access, the access is allowed. Otherwise a protection violation occurs, and the user job is denied access to the file. 3. Many systems recognize three classifications of users in connection with each file for access control. 1. Owner- User who created the file. 2. Group- Set of users who are sharing the file and need similar access is a Group. 3. Universe- All other users in the system constitute the universe.. Summary A file is an abstract data type defined and implemented by the operating system. It is a sequence of logical records. A logical record may be a byte, a line or more complex data item. The operating system may specifically support various record types or may leave that support to the application program. The major task for the operating system is to map the logical file concept onto physical storage devices such as magnetic tape or disk. Since the physical record size of the device may not be the same as the logical record size, it may be necessary to block logical records. Again this task may be supported by the operating system or left for the application program. A single level directory in a multi user system cause naming problems, since each file must have a unique name. A two-level directory solves this problem by creating a separate directory for each user. A tree-structured directory allows the user to create sub directories to organize his files. An Acyclic-graph directory structure allows sub directories and files to be shared, but complicate in searching and deletion.

19 A general-graph directory allows complete flexibility in the sharing of files and directories, but some times require garbage collection to recover unused disk space. File system can be mounted into the system s naming structures to make them available once mounted; the files with in the partition are available for use. We also discussed about protection of files by providing access control them. Key Terms File attributes-file operations-file types-sequential access-direct access-device directory-mount point-access Control. Key Term Quiz 1. path name begins at the root and follow a path down to the specified file, giving the directory names on the path. 2. path name defines a path from the current directory 3. is the empty directory at which the mounted file system will be attached. 4. FTP uses both and access. 5. The sequence of directories searched when a file is named is called the. Multiple Choice Questions 1. Repositioning with in a file is called as a. Seek b. Fetch c. Truncate d. Abort. 2. Which of the following file name extension suggests that the file is Backup copy of another file? a. TXT b. COM c. BAS d. BAK

20 3. A tree structured file directory system a. Allows easy storage and retrieval of file names b. Is a much debated unnecessary feature c. Is not essential when we have millions of files d. None of the above 4. The number provided by the user to the operating system is normally called as a. Relative block number b. Process control block number c. Process Identifier d. None of the above 5. Truncating a file does the following operation. a. Keeps the attributes to remain unchanged b. Deletes the attributes c. Updates the content of the file d. None of the above. 6. Which of the following information is contained in inode structure? a. The file size b. The name of the owner of the file c. The access permissions for the file d. All the dates of modification since the file s creation e. The number of symbolic links for this file 7. File which are linked have as many inodes as are the links. a. True b. False 8. Which directory under the root contains the information on devices? a. /usr/bin b. /usr/sbin c. /usr/peripherals/dev d. /etc/dev 9. Absolute path names begin by identifying path from the root. a. True b. False 10. The file s properties in Windows environment include which amongst the following: a. File owners name b. File size c. The date of last modification d. Date of file creation e. The folder where it is located

21 OBJECTIVE TYPE QUESTIONS PART-A 1. What is a file? 2. List the various file attributes. 3. What are the various file operations? 4. What are the information associated with an open file? 5. What are the different accessing methods of a file? 6. What is Directory? 7. What are the operations that can be performed on a directory? 8.What are the most common schemes for defining the logical structure of a directory? 9. Define UFD and MFD. 10. What is a path name? 11. What are the various layers of a file system? PART-B 1. What are files and explain the access methods for files? File definition Attributes, operations and types Sequential access with diagram Direct access Other access methods-index with diagram 2. Explain the schemes for defining the logical structure of a directory. Single level directory with diagram Two level directory with diagram Tree structured directory with diagram Acyclic-graph directory with diagram General graph directory with diagram