Transcript Document
Princess Nora University Faculty of Computer & Information Systems Computer science Department Operating Systems (CS 340 D) Dr. Abeer Mahmoud (Chapter-11) File System Interface Chapter 11: File System Interface 1. File Concept 2. Access Methods 3. Directory Structure 4. File Sharing 5. Protection 3 Dr. Abeer Mahmoud OBJECTIVES: 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 4 Dr. Abeer Mahmoud Background 5 Dr. Abeer Mahmoud The file system consists of two distinct parts: 1. a collection of files, each storing related data, and 2. a directory structure, which organizes and provides information about all the files in the system. 6 Dr. Abeer Mahmoud File Concept A file is a named collection of related information that is recorded on secondary storage. Commonly, files represent programs (both source and object forms) and data. Data files may be numeric, alphabetic, alphanumeric, or binary. Files may be free form, such as text files, or may be formatted rigidly. The information in a file is defined by its creator. 7 Dr. Abeer Mahmoud File Concept (cont.) Many different types of information may be stored in a file source programs, object programs, executable programs, numeric data, text, payroll records, graphic images, sound recordings, ….. A file has a certain defined structure, which depends on its type. o o o o 8 e.g: A text file is a sequence of characters organized into lines A source file is a sequence of subroutines and functions, each of which is further organized as declarations followed by executable statements. An object file is a sequence of bytes organized into blocks understandable by the system’s linker. An executable file is a series of code sections that the loader can bring into memory and execute. Dr. Abeer Mahmoud File Structure 9 Dr. Abeer Mahmoud File Structure None - sequence of words, bytes Simple record structure Lines Fixed length Variable length Complex Structures Formatted document Relocatable load file Can simulate last two with first method by inserting appropriate control characters Who decides: Operating system Program 10 Dr. Abeer Mahmoud File Attributes Name – only information kept in human-readable form Identifier – unique tag (number) identifies file within file system 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 Information about files are kept in the directory structure, which is maintained on the disk 11 Dr. Abeer Mahmoud File Operations 12 Dr. Abeer Mahmoud File Operations File is an abstract data type Create Two steps are necessary to create a file. Write To write a file, we make a system call specifying both the name of the file and the information to be First, space in the file system must written to the file. be found for the file how? Second, an entry for the new file must be made in the directory. 13 Given the name of the file, the system searches the directory to find the file’s location. The system must keep a write pointer to the location in the file where the next write is to take place. The write pointer mustbe updated whenever a write occurs. Dr. Abeer Mahmoud File Operations (cont.) Read Delete To read from a file, we use a system call To delete a file that Provide name and search the 1. specifies file name and where (in directory for it memory) the next block of the file Having found it, we release all file should be put. space, so that it can be reused by 2. the directory is searched for the other files, and erase the directory associated entry, entry 3. the system keeps a read pointer to the location in the file where the next read is to take place. 4. Once the read has taken place, the read pointer is updated 14 Dr. Abeer Mahmoud File Operations (cont.) Truncating a file The user may want to erase the contents of a file but keep its attributes. Rather than forcing the user to delete the file and then recreate it, this function allows all attributes to remain unchanged— except for file length—but lets the file be reset to length zero 15 Dr. Abeer Mahmoud File Types – Name, Extension 16 Dr. Abeer Mahmoud File Access Methods 17 Dr. Abeer Mahmoud Access Methods 1. Sequential Access 2. Direct Access (or relative access) Information in the file is A file is made up of fixed length processed in order, one of logical records that allow record after the other. programs to read and write records rapidly in no particular This mode of access is by order. far the most common read next, write next reset , no read after last write (rewrite) 18 read n, write n , position to n read next, write next , rewrite n n = relative block number Dr. Abeer Mahmoud Simulation of Sequential Access on a Directaccess File 19 Dr. Abeer Mahmoud Directory and Disk Structure 20 Dr. Abeer Mahmoud Directory Structure A storage device can be used in its entirety for a file system. It can also be subdivided into partitions (each partition can hold a file system) Partitioning is useful for limiting the sizes of individual file systems, Partitions are also known as slices or (in the IBM world) minidisks. Any entity containing a file system is generally known as a volume. 21 Dr. Abeer Mahmoud Directory Structure (cont..) Each volume can be thought of as a virtual disk. Each volume that contains a file system must also contain information about the files in the system. This is the device directory (simply as that directory) records information—such as name, location, size, and type—for all files on that volume. file-system organization. 22 Dr. Abeer Mahmoud Directory Structure (cont..) A collection of nodes containing information about all files Directory Files F1 F2 F3 F4 Fn Both the directory structure and the files reside on disk Backups of these two structures are kept on tapes 23 Dr. Abeer Mahmoud A Typical File-system Organization 24 Dr. Abeer Mahmoud Operations Performed on Directory Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system 25 Dr. Abeer Mahmoud Organize the Directory (Logically) to Obtain Efficiency – locating a file quickly Naming – convenient to users most common schemes for defining the logical structure of a directory. Single-Level Directory Two-Level Directory Tree-Structured Directories 26 Dr. Abeer Mahmoud Single-Level Directory A single directory for all users The simplest structure Limitation Naming problem: when the number of files increases or when the system has more than one user. Since all files are in the same directory, they must have unique names e.g :The MS-DOS operating system allows only 11-character file names; UNIX, in contrast, allows 255 characters. Grouping problem : Keeping track of so many files is unaffordable 27 Dr. Abeer Mahmoud Two-Level Directory Separate directory for each user Path name Can have the same file name for different user Efficient searching No grouping capability 28 Dr. Abeer Mahmoud Tree-Structured Directories Efficient searching 29 Grouping Capability Dr. Abeer Mahmoud File Share and Protection 30 Dr. Abeer Mahmoud 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 filesharing method 31 Dr. Abeer Mahmoud File Sharing – Multiple Users User IDs identify users, allowing permissions and protections to be per-user Group IDs allow users to be in groups, permitting group access rights 32 Dr. Abeer Mahmoud Protection File owner/creator should be able to control: what can be done by whom Types of access 33 Read Write Execute Append Delete List Dr. Abeer Mahmoud Thank you End of Chapter 11 34 Dr. Abeer Mahmoud