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Published byEstella Chrystal Price Modified over 9 years ago
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It consists of two parts: collection of files – stores related data directory structure – organizes & provides information Some file systems may have third part: partitions – used to separate physically & logically large collection of directories File protection – multiple users access the file
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File Concept Access Methods Directory Structure File-System Mounting File Sharing Protection
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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
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Storage media : magnetic disk, magnetic tapes, optical disks…. Os provides logical view of information storage It abstracts from the physical properties of its storage devices to define a logical unit (file) Files are mapped by os onto physical devices Storage devices are non volatile – persistent through power failures File – a named collection of related information that is recorded on the secondary storage User s perspective – smallest allotment of logical storage
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Contiguous logical address space Types: ◦ Data files numeric Character alphanumeric binary ◦ Program files ◦ Text files (free forms)
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Information in the file is defined by its creator sequence of words, bytes, bits, lines & records (source, object, pay roll, text, graphics) 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
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Text files – sequence of characters organized into lines Source files – sequence of subroutines & lines, further organized into executable statements Object file – sequence of bytes organized into blocks understandable by system linker\ Executable file – series of code sections that the loader can bring into memory & execute
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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
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File is an abstract data type Operations on file: Create – space in file system & entry for new file Write – name of the file & information to be written on the file Read – name of the file & read pointer Reposition within file – file seek (repositioning within a directory) Delete – name of the file Truncate – erasing the contents but keeping the attributes File table: Open(F i ) – search the directory structure on disk for entry F i, and move the content of entry to memory Close (F i ) – move the content of entry F i in memory to directory structure on disk
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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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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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Macintosh os- supports a minimal no of file structure It expects files to contain 2 parts: resource fork – information of interest to user data fork – program code or data
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Physical record size will exactly match the length of the desired logical record Logical records may even vary in length Packing – no.of logical records into physical blocks Example – unix files – stream of bytes file system automatically packs & unpacks bytes into physical disk blocks – 512 bytes per block Conversion of physical to logical is a simple sw problem disk space – always allocated in blocks some portion of last block – generally wasted wasted bytes allocated to keep everything in units of blocks is internal fragmentation Block size is directly proportional to internal fragmentation
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