File Systems

CSCI 4534

What is a file?

• A file is information that is stored on disks or other external media

Why do we use files?

• We often need for several applications or users to share data

• That data needs to be stored and retrieved

What is a file system?

• The part of the operating system that deals with files is called a file system

• This section discusses how files are structured, named, accessed, used, protected, and implemented

File Systems

• Consist of two parts– A collection of files with stored data

– A directory structure• With organization of and information about

all the files in the system

• There may be a third part– Partitions which separate directories

File Concepts

• The Operating System provides a uniform logical view of how information is stored– Physical: stored on nonvolatile media– Logical: named collection of related data

• Types of files– Data: numeric, alphabetic, alphanumeric,

binary, ASCII– Programs: source, object, executable

File Concepts

• Meaning of a file is defined by the creator (or the user?)

• Files have structure according to their type– Text: sequence of characters organized into lines

– Source: sequence of subprograms consisting of data and instructions

– Object: sequence of bytes organized into blocks understandable by linker

– Executable: binary code and data to be loaded into memory

and executed

File Types

• Regular files: contain user information– ASCII – good for being edited with a text editor– Binary – have an internal structure that is

meaningful to the programs that use them

• Directories: system files for maintaining the structure of the file system

File Attributes

• Typical Attributes – see also Figure 6-4

– Name– Type– Location– Size – Protection and access control– Time, date and user ID: for creation, last

modification, last use

File Operations

• System calls are provided for:– Create a file: find space, add to directory

– Delete: release file space, delete dir entry

– Open: system fetches attributes and disk addresses

– Close: space is freed

– Read: name and location

– Write: name and data

– Append: add data to end of file

– Seek: repositions file pointer

File Operations

• Other operations– Get attributes: see Make p. 388

– Set attributes: e.g. protection modes

– Rename– Copy

File Operations

• Before files are used, an open( ) system call is usually made– Avoids searching for the file each time– O.S. keeps a table about all open files

• May have a count of users if shareable

– File name is associated with index into table, open( ) returns a pointer (we have seen an int)

– The open( ) call can specify access-mode information and can check for permissions

File Operations

• What happens when a file (such as in UNIX) is shared by several users?– The O.S. uses two tables

• A system-wide table: location, access dates, size

• A per-process table– Current file pointer for this process

– Each entry points to the system-wide table

File Types

• What file types should an O.S. recognize?– See types in Figure 6-1

• File names are followed by an extension– Extension specifies type of file and type of

operations that can be done on the file• .com, .bat, .exe can all be executed

• .asm is extension that an assembler is expecting

File Structure

• O.S.s may support different types of files– Some may support many or a few– However, all must support executable files

• Files can be seen as:– A sequence of 8-bit bytes

• no interpretation is made; flexible

– A sequence of records– A tree (see Figure 6-2)

File Structure

• How can a specific logical record in a file be found?

• Disk I/O uses blocks– All blocks are the same size– Logical records are packed into physical blocks– In UNIX, files are simply a stream of bytes– Those bytes are stored into a specific # of bytes

per block (eg. 512, 1024)

File Structure

• What determines how many logical records are in each block?– Logical record size– Physical block size

• A file, then, is a sequence of blocks– Some part of the last block is wasted (internal

fragmentation)

Access Methods

• Sequential Access– One record is processed after the other

• Examples: compilers, editors

• Mostly reading and writing is done

• Works on sequential-access devices and on random-access devices

– Operations: read next, write next

Access Methods

• Random Access– Records are accessed by key, not position– File seen as a #ed sequence of blocks or records– Useful in databases

• Provides immediate access to large amounts of data

– Where to start reading?• Each Read operation gives reading position• Or Seek finds the first position, then reads

sequentially from there

Directory Structure

• File systems are organized:– Disks are split into one or more partitions

• IBM calls them minidisks, others are called volumes• Usually each disk has a partition that stores files and

directories

– Each partition contains information about the files within it

• Called a device directory or a volume table of contents which holds the file attributes for files on this partition

Directory Structure

• The directory can be viewed as a symbol table– Translates file names onto directory entries

• What do we want to do with directories?– Search for a file– Create, delete– Rename, list files– Traverse the file system

Directory Structure

• Single-Level Directory– All files have unique names

• Two-Level Directory– Each user has a UFD (user file directory)– A higher-level MFD (master) is indexed by

user name or account number

Directory Structure

• Hierarchical Directories– Must specify pathname

• Absolute: begins at root

• Relative: begins at current directory

– Directory contains pointers to files (0) or subdirectory (1)

– Must have current directory, be able to change directories, be able to delete directories

– Allow access to other users’ files if given access

Directory Operations

• Create• Delete• Opendir• Closedir• Readdir• Rename• Link

File-System Implementation

• Layout (see Figure 6-11)

– Master Boot Record– Boot Block– Superblock

File-System Implementation

• Contiguous Allocation

• Linked List Allocation

• File Allocation Table

• I-nodes

Implementing Directories

• Directories provide information to find disk blocks– Disk address of file – contiguous – Number of the first block – linked list schemes– Number of the i-node

• Where are attributes stored?– In directory entry– In the i-nodes

• See Figure 6-17 for ways to handle long file names in directories

File Sharing

• Directed Acyclic Graph (DAG)

File Protection

• We want to keep information safe from:– physical damage (reliability)– improper access (protection)

• We need protection for these operations:– Read, write, execute, append, list, delete

File Protection

• Access control

• Use an access control list– specifies user name and types of access allowed– O.S. checks access list when a request is made– lists are tough to maintain

• Three classifications:– Owner, Group, Universe– Example of Sara