Lecture Operating systems: A concept-based approach (2/e): Chapter 7 - Dhananjay M. Dhamdhere
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Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 1
Copyright © 2008
Goals of a File system
•
Convenient and fast access to files
– Different file organizations to suit different user requirements
•
Reliable storage of files
– Prevents damage due to crashes, etc.
•
Controlled sharing of files
– Unauthorized persons should not be able to access files
•
…. also ensure efficient use of I/O devices
– We discuss this aspect in Chapter 12
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 2
Copyright © 2008
Structure of a File system
•
A file system is structured into a hierarchy of layers
– File system layer
* Deals with files as objects
Creates files, allocates disk space to them
Implements file sharing and prevents unauthorized access
Prevents damage due to crashes
– Input-Output Control System (IOCS) layer
* Implements file system operations, ensuring
Efficient access to records in files
Efficient operation of I/O devices
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 3
Copyright © 2008
File system and IOCS layers
• Each layer contains policy and mechanism modules
• A mechanism module invokes modules of the lower layer
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 4
Copyright © 2008
File system
•
A Directory
– Groups a set of files
* It contains an entry for each of these files
* The entry contains information useful for accessing the file
– Users have different directories
* Helps to separate files of different users
* Provides file naming freedom
– Directories are organized in a hierarchical structure
* Permits a user to organize her files logically, e.g., according to
activities
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 5
Copyright © 2008
Logical organization in file systems
• Two files named beta exist. The file system must open the correct
one when a process executes open (beta, .. )
• Records may be organized differently in different files
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 6
Copyright © 2008
Logical organization in file systems
•
Comments on the previous slide
– Two files named beta exist in the file system
* Thus users enjoy file naming freedom
– Processes Pi and Pj share one of these files
* The rules of sharing are determined by file sharing semantics
– Files beta and phi have different organizations and are accessed
differently
* File beta is a sequential file; its records are read in a sequence
* File phi is a direct file; its records can be read in random order
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 7
Copyright © 2008
Overview of file processing
•
Implementation of file processing activities
– File processing activities are implemented through combined
actions of a compiler, modules of the file system and IOCS, and
the kernel
* The compiler replaces open and read/write statements in a program
with calls on file system and IOCS modules
These modules implement the desired file operations
These file system and IOCS modules are linked to the program
* A process executing the program invokes these modules during its
operation to perform file operations
The modules may make system calls to invoke functionalities of
the kernel
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 8
Copyright © 2008
File processing in a program
Source program
compiled program
linked program
• The program invokes file system modules during execution
• File system modules invoke the IOCS modules
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 9
Copyright © 2008
File Types
•
Files can be of two types
– Structured files
* A File contains records
* Each record contains a key field
Key fields of records in a file contain unique values
* Records in a file can be accessed in different ways
Sequential access
Random access by specifying the key value of a record
– Byte-stream files
* A file is merely a stream of bytes
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 10
Copyright © 2008
Records and fields in a file
• File employee_info contains records for employees
• A record contains fields for employee no, name, etc.
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 11
Copyright © 2008
File operations performed by processes
•
File operations
– Opening a file
* FS opens the file only if user possesses necessary privileges
– Reading or writing a record
* FS considers organization of the file and performs the operation
accordingly
– Closing a file
* FS updates information concerning file size in its directory entry
– File creation, deletion and renaming
– Specifying who can access a file
* The information is entered in the file’s directory entry
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 12
Copyright © 2008
File organizations
•
Record access pattern
– Definition
* The order in which a process accesses records in a file
– Two fundamental record access patterns in applications
* Sequential access
A process always performs an operation on the next record
Suits batched operations; e.g., payroll
* Random access
A process may access any record in a file
– File system should effectively support the record access pattern
of an application
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 13
Copyright © 2008
File organizations
•
File organization
– Method of organizing and accessing data in a file
– Sequential file organization
* Records in the file are accessed sequentially
This file organization is independent of characteristics of I/O
devices
– Direct file organization
* Records are accessed randomly—a record is found using an
address calculation formula applied to the value in its key field
Files tend to be device dependent; may contain dummy records
– Index sequential file organization
* File has a hierarchical organization. To locate a record, a region in
the file is first located and then searched sequentially
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 14
Copyright © 2008
Sequential and direct files
• In sequential files, an operation is always performed on the
next record
• In direct files, an address calculation formula computes
location of a record from its key value
• Dummy records have to exist for the formula to work
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 15
Copyright © 2008
An Index Sequential file
• The higher level index is searched to find a group of tracks
where a record may exist
• The track index is searched to find the track which may
contain the record
• The track is searched to check whether the record exists
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 16
Copyright © 2008
A typical directory entry
• Location info indicates where records of the file are located
• Protection info indicates who can access the file and in
what manner
• Flags indicate whether the file is itself a directory, etc.
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 17
Copyright © 2008
Two-level directories: Master and user directories
• A separate user directory exists for each user. This arrangement
provides file naming freedom
• A user can access files of other users by going to that
user’s directory through the master directory
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 18
Copyright © 2008
Generalizations of the two-level directory structure
•
Generalization provides several benefits
– Directory as a file
* A directory can exist in another directory
* It leads to a directory hierarchy
* User can form groups and subgroups of files
– Generalized syntax for accessing files
* A path name permits any file in the directory hierarchy to be
accessed (subject to access privileges); e.g., ../projects/alpha
•
Accessing a file
– Each user has a home directory and a current directory
– A file is accessed using absolute or relative path names
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 19
Copyright © 2008
Sample directory hierarchy
• A rectangle represents a directory, a circle represents a file
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 20
Copyright © 2008
Directory graphs
•
Links
– A link provides an alternative method of naming a file
* It associates a name in a specified directory with a file that may
exist anywhere in the directory hierarchy
* It can be used to provide a shortcut to access the file
– Use of links makes a directory hierarchy into a graph
* Alternative path names exist for the same file
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 21
Copyright © 2008
Links
• link (~B, ~A/projects/beta, showcase) creates a link named
‘showcase’ from directory B to file ~A/projects/beta
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 22
Copyright © 2008
Mounting of a file system
•
An OS may have several file systems
– Each file system has a separate directory hierarchy
– Directory hierarchy of two file systems can be combined through
mounting
– A mount operation ‘attaches’ a file system to a directory in
another file system
* Mounting is typically a privileged operation
* It has a temporary effect that lasts until an unmount operation is
performed
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 23
Copyright © 2008
Mounting of a file system
(a) Directory hierarchies of two file systems
(b) Command mount (meeting, ~A/admin) combines them as shown
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 24
Copyright © 2008
Interface between the file system and IOCS
•
The file system and the IOCS jointly implement
operations on files
– A file system creates, deletes and protects files
* It allocates disk space to a file
* It permits a user to specify protection information
– The file system also performs opening and closing of files
* It performs path name resolution at open; it also checks whether the
user is authorized to access the file
– File operations are performed by the IOCS
•
The file control block (FCB) is the interface between the
file system and the IOCS
Chapter 7:
File Systems
Dhamdhere: Operating Systems—
A ConceptBased Approach, 2ed
Slide No: 25
Copyright © 2008
