Lecture Operating system principles - Chapter 12: File management

After studying this chapter, you should be able to: Describe the basic concepts of files and file systems, understand the principal techniques for file organization and access, define B-trees, explain file directories, understand the requirements for file sharing.

Chapter 12 File Management

• Files are the central element to most

applications

– file as an input to applications

– file as an output for long-term storage and for later access

• Desirable properties of files:

– Long-term existence

– Controlled sharing between processes

– Structure that is convenient for particular

applications

File Structure Fields and Records

• Fields

– Basic element of data

• e.g., student’s last name

– Contains a single value

– Characterized by its length and data type

• Records

– Collection of related fields

• e.g., a student record

– Treated as a unit

File Structure File and Database

• File

– Collection of similar records

– Treated as a single entity and may be

referenced by name

– Access control restrictions usually apply at the file level

• Database

– Collection of related data

– Explicit relationships exist among elements

– Consists of one or more files

A Big Picture

How to identify and

locate a selected file?

How to enforce user

access control in

shared systems?

How to organize records as a sequence of blocks

for I/O?

individual block I/O requests must be scheduled for optimizing performance

How to organize

records in a file and

access a particular

record in a file?

File Organization

• The basic operations that a user or

application may perform on a file are

performed at the record level

– The file is viewed as having some structure

that organizes the records

• File organization refers to the logical

structuring of records

– Determined by the way in which files are

accessed (access method)

Criteria for File Organization

• Important criteria include:

– Short access time

– Ease of update

– Economy of storage

– Simple maintenance

– Reliability

Criteria for File Organization

• Priority will differ depending on the use

– For batch mode file processing, rapid access for retrieval of a single record is of minimal

concern

• These criteria may conflict

– Use of indexes (conflict with economy of

storage) can be a primary means of

increasing the speed of access to data

The Pile

• Data are collected in the

order they arrive

– No structure

• Purpose is to accumulate a

mass of data and save it

• Records may have different

fields

– field should be self-describing (field

name + value)

– field length should be known (delimiters,

subfield or default for a field type)

The Pile

• Record access is by exhaustive search

• Used when data are collected and stored

prior to processing or data are not easy to organize

•  Uses space well when data vary in size

The Sequential File

• Fixed format used for records

• Records are of the same length

– same number of fixed-length fields

in a particular order

• Only the values of fields need to

be stored

• Field name and length are

attributes of the file structure

The Sequential File

• Key field

– Uniquely identifies the record

– Records are stored in key sequence

•  Optimal for batch applications if they involve

the processing of all the records

•  Easily stored on tape and disk

•  Poor performance for interactive applications

– considerable processing and delay due to the

sequential search of the file for a key match

Indexed Sequential File

• An index is added to support

random access

– An index record contains a key

field and a pointer into the main file

– The index is a sequential file

– For searching

• Search the index to find the highest key value that is equal to or precedes the desired key value

• Search continues in the main file at the location indicated by the pointer

Indexed Sequential File

Example

• Consider searching a particular key value

in a sequential file with 1 million records

• An overflow file is added

• A new record is added to the overflow file and is

located by following a pointer from its predecessor

record

• The indexed sequential file is occasionally merged

with the overflow file in batch mode

• Greatly reduces the time required to access a

single record, without sacrificing the sequential

nature

Indexed Sequential File

– allows variable-length records

• Uses multiple indexes for different key

fields

– An exhaustive index contains one entry for

every record in the main file

– A partial index contains entries to records

Indexed File

• When a new record is added to the main file, all

of the index files must be updated

• Used mostly in applications where

– timeliness of information is critical and

– data are rarely processed exhaustively

– examples: airline reservation systems and inventory

control systems

Directory Elements

• Basic Information

– File name: must be unique

– File type: e.g., text, binary

– File organization

• Address Information

– Volume: device on which file is stored

– Starting address: e.g., cylinder, track on disk

– Size used: in bytes, words or blocks

– Size allocated: maximum size of the file

Directory Elements

• Access Control Information

– Owner: able to grant/deny access to other users and

to change these privileges

– Access information: e.g., user’s name and password

for each authorized user

– Permitted actions: controls reading, writing, executing, transmitting over a network

• Usage Information

– Date Created, Identity of Creator, Date Last Read

Access, Identity of Last Reader, Date Last Modified

Hierarchical, or Tree-Structured Directory

• Master directory with user

directories underneath it

• Each user directory may

have subdirectories and

Hierarchical, or Tree-Structured Directory

•  Easily enforce access restriction on

directories.

•  Easily organize collections of files

•  Minimize the difficulty in assigning

unique names.

• The tree structure allows users to find a

file by following a path from the root or

master directory down various branches

until the file is reached

• The series of directory names, culminating

in the file name itself, constitutes a

pathname for the file

• Duplicate filenames are possible if they

have different pathnames

– Files are referenced

relative to the working

directory unless an

explicit full pathname

is used

File Sharing

• In multiuser system, there is almost

always a requirement for allowing files to

be shared among a number of users

• Two issues

– Access rights

– Management of simultaneous access

Access Rights

• A wide variety of access rights have been

used by various systems

– often as a hierarchy, with each right implying

those that precede it

– User can only determine that the file exists

and who its owner is

Access Rights cont…

• Execution

– The user can load and execute a program but cannot copy it, e.g., proprietary programs

• Reading

– The user can read the file for any purpose,

including copying and execution

• Appending

– The user can add data to the file but cannot

modify or delete any of the file’s contents

Access Rights cont…

Simultaneous Access

• When access is granted to append or

update a file to more than one user, the

OS or file management system must

enforce discipline

• User may lock the entire file or individual

records during update

• Mutual exclusion and deadlock are issues for shared access, ref readers/writers

problem

Blocks and records

• Records are the logical unit of access of a

structured file

• Blocks are the unit for I/O with secondary storage

• For I/O to be performed, records must be

organized as blocks

• Three methods of blocking are common

– Fixed length blocking

– Variable length spanned blocking

Fixed Blocking

• Fixed-length records are used, and an

integral number of records are stored in a

Fixed Blocking

Variable Length Spanned Blocking

• Variable-length records are used and are

packed into blocks with no unused space

• Some records may span multiple blocks

– Continuation is indicated by a pointer to the

successor block

•  Efficient for storage and does not limit

the size of records

Variable Blocking:

Spanned

•  Difficult to implement

•  Records that span two blocks require

two I/O operations

Variable-length unspanned blocking

• Uses variable length records without

spanning

•  Wasted space in most blocks because

of the inability to use the remainder of a

block if the next record is larger than the

remaining unused space

•  Limits record size to the size of a block

Variable Blocking:

Unspanned

Revisit the Big Picture

Describes the location

of all files plus their

attributes

Only authorized users

are allowed to access

particular files in

particular ways

Records must be organized as a sequence of blocks for output and unblocked after

input

individual block I/O requests must be scheduled for optimizing performance

User views the file as

having some structure

that organizes the

records; different

access methods reflect

different file structures