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Rose-Hulman CSSE 332 - File Management

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Why a file system?Why a file system There is a general need for long-term and shared data storage: need to store large amount of information persistent storage (outlives process and system reboots) concurrent sharing of information Files meet these requirements The file manager or file system within the OS Set of system software proving services to users and applications in use of files Files are accessed through file management system2Concept of a file A named collection of related data stored on secondary storage File name may encode the file type file extensions in UNIX and Windows Abstraction presented to the user Common examples of File types Regular files, directories Executable files special files (block and character) Archives3File structure (logical) None - sequence of words, bytes Simple record structure Lines, Fixed length, Variable length Complex Structures Formatted document, multi-media documents Who decides: Operating system Application DBMS4File attributes Name – only information kept in human-readable form 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.5File operations create write read reposition within file – file seek delete truncate open(file_i) – search the directory structure on disk for entry file_i, and move the content of entry to memory. close (file_i) – move the content of entry file_i from memory to directory structure on disk.6Examples of file types7File Type Usual extension Functiion Executable exe, com, bin or none ready-to-run machine-language program Object obj, o complied, machine language, not linked Source code c, p, pas, 177, asm, a source code in various languages Batch bat, sh commands to the command interpreter Text txt, doc textual data documents Word processor wp, tex, rrf, etc. various word-processor formats Library lib, a libraries of routines Print or view ps, dvi, gif ASCII or binary file Archive arc, zip, tar, rar related files grouped into one file, sometimes compressed.Access methods for file data Sequential Access – (most common) read next  write next , (rewrite) reset no read after last write Direct Access: n = relative block number (on disk) read n write n, rewrite n position to n read next write next 8Directory structure Directory contains a collection of nodes containing information about all files Both the directory structure and the files reside on disk Backups of these two structures are kept on tapes9F 1F 2F 3F 4F nDirectoryFilesInformation per file in a directory Name  Type Address  Current length Maximum length Date last accessed (for archival) Date last updated (for dump) Owner ID (who pays) Protection information (discuss later)10Directory operations Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system11Organize the directory (logically) 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, …)12Single-level directory13 A single directory for all users.• Naming problem• Grouping problemTwo-level directory14 Separate directory for each user.• Path name• Can have the same file name for different user• Efficient searching• No grouping capabilityTree-structured directories15Tree-structured directories Efficient searching Grouping Capability Absolute or relative path name Current directory (working directory) cd /Users/faculty/defoe/Public/ type echo Creating a new file is done in current directory Creating a new subdirectory is done in current directory16Protection File owner/creator should be able to control: what can be done by whom Types of access Read Write Execute Append Delete List17Access lists and groups18 Mode of access: read, write, execute Three classes of usersRWXa) owner access 7 1 1 1RWXb) groups access 6 1 1 0RWXc) public access 1 0 0 1 Ask manager to create a group (unique name), say G, and add some users to the group. For particular file or subdirectory, define an appropriate access.owner group publicchmod 761 gameAttach a group to a filechgrp G gameFilesystem structure Disk divided into one or more partitions independent file system on each partition Sector 0 contains the Master Boot Record (MBR) MBR contains partition table one partition marked as active boot block – first block of active partition  BIOS reads and executes MBR, which reads boot block and executes it program in boot block loads OS and runs it Often file system contains superblock which contains key file system parameters 19Example disk and filesystem layout20boot block super block inode list root dirMBRPartition Tablepartition 1partition 2 (active)partition 3free space management files & dirsFile allocation on disk Disk is divided into blocks or sectors Files are stored on secondary storage in blocks or sectors Blocks are the unit of I/O transfer with secondary storage (on LINUX and UNIX, sectors on Windows) Blocks can be of fixed length or variable-length Need file allocation table (FAT) to keep track of files on disk Each file has a FAT entry21File allocation methods  Keep track of which sectors/blocks on the disk belong to which logical file/directory. Methods include Contiguous allocation Linked (chained) allocation Indexed allocation22Contiguous allocation Each file occupies a set of contiguous blocks on the disk Simple FAT entry only starting location (block #) and  length (number of blocks) are required Random access Wasteful of space  external fragmentation,  may use compaction to fix Files cannot grow Pre-allocation of blocks is required Maximum file size is known in advance23Contiguous allocation24What happens if file F


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Rose-Hulman CSSE 332 - File Management

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