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

File Concept Access Methods Directory Structure File-System Mounting File Sharing Protection

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

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

Contiguous logical address space

Types: ◦ Data files

numeric Character alphanumeric binary

◦ Program files◦ Text files (free forms)

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

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

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

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(Fi) – search the directory structure on disk for entry Fi, and

move the content of entry to memory Close (Fi) – move the content of entry Fi in memory to directory

structure on disk

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

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

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

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 bytesfile system automatically packs & unpacks bytes into physical disk

blocks – 512 bytes per block

Conversion of physical to logical is a simple sw problemdisk space – always allocated in blockssome portion of last block – generally wastedwasted bytes allocated to keep everything in units of blocks is internal fragmentationBlock size is directly proportional to internal fragmentation