1 Chapter 12 File Management Overview File organisation and Access File Directories File Sharing...

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Chapter 12File Management

• Overview• File organisation and Access

• File Directories

• File Sharing

• Record Blocking

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Files

• 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

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File StructureFields 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

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File StructureFile 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

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Roadmap

• Overview

• File organisation and Access• File Directories

• File Sharing

• Record Blocking

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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)

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Criteria for File Organization

• Important criteria include:

– Short access time

– Ease of update

– Economy of storage

– Simple maintenance

– Reliability

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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

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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 length should be known

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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 and structure

• Adequate for exhaustive searches• Easy to update• Unsuitable for most applications

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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

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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

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Indexed Sequential File

• An index is added to support random access– The index is a sequential file

– An index record contains a key field and a pointer into the main 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

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• 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

Indexed Sequential File

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Indexed File• Records are accessed only through

their indexes– no restriction on the placement of

records

– 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

where the field of interest exists

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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

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Roadmap

• Overview

• File organisation and Access

• File Directories• File Sharing

• Record Blocking

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File Directory

• Contains information about files

– Attributes

– Location

– Ownership

• Directory itself is a file owned by the

operating system

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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

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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

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Hierarchical, or Tree-Structured Directory

• Master directory with user directories underneath it

• Each user directory may have subdirectories and files as entries

• Each directory and subdirectory can be organized as a sequential file

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Naming

• 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

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Naming

• Usually an interactive user or a process is associated with a current or working directory– Files are referenced

relative to the working directory unless an explicit full pathname is used

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Roadmap

• Overview

• File organisation and Access

• File Directories

• File Sharing• Record Blocking

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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

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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.

• None– User may not even know of the files existence

• Knowledge– User can only determine that the file exists

and who its owner is

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Access Rights cont…

• Execution– The user can load and execute a program but

cannot copy it

• 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

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Access Rights cont…

• Updating– The user can modify, delete, and add to the

file’s data.

• Changing protection– User can change access rights granted to

other users

• Deletion– User can delete the file

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User Classes

• Owner– Usually the files creator, has full rights and may

grant rights to others

• Specific Users– Individual users who are designated by user ID

• User Groups– A set of users identified as a group

• All– All users who have access to this system

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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

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Roadmap

• Overview

• File organisation and Access

• File Directories

• File Sharing

• Record Blocking

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Blocks and records

• Records are the logical unit of access of a structured file

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

• Three methods of blocking are common– Fixed length blocking– Variable length spanned blocking– Variable-length unspanned blocking

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Fixed Blocking

• Fixed-length records are used, and an integral number of records are stored in a block

• Unused space at the end of a block is internal fragmentation

• Common for sequential files with fixed-length records

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Fixed Blocking

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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

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Variable Blocking: Spanned

• Difficult to implement

• Records that span two blocks require two I/O operations

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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

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Variable Blocking: Unspanned

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A 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