File management - Chap 12 - OS

8/10/2019 File management - Chap 12 - OS

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2014 by Greg Ozbirn, UTD, foruse with Stalling's 8th Ed. OS book

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Chapter 12

File Management

Fall 2014


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Files and File Systems

The file system is one of the most important parts of anoperating system.

Some desirable properties of files include:

Long-term existence: kept on secondary storage

Sharable between processes: for shared access

Structure: some file systems manage the internal

structure of a file, which can be organized in a way

suitable for particular applications


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File Operations

Create creates a new file and adds it to the directory

Delete

remove from the directory and destroyed

Open allows a process to perform functions on the file

Close

the process is no longer allowed to operate on the file

Read read all or a portion of the file

Write

add new data or change existing data in the file


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File Structure

Field:

Basic element of data

Contains a single value

Characterized by its length and data type

Can be fixed length or variable length

Record:

Collection of related fields

Treated as a unit Example: employee record

Can be variable length if some of the fields are variable

length, or if the number of fields may vary


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File Structure

File

Collection of similar records

Treated as a single entity

Have unique file names

May restrict access

Database

Collection of related data (one or more files)

Relationships exist among elements Managed by separate DBMS


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Typical File Operations

Retrieve_Allto process the whole file

Retrieve_Oneselects a single record

Retrieve_Nextmove forward in file

Retrieve_Previousmove backward in file

Insert_Onemay require adding at a particular position

Delete_Onemay need to preserve sequence

Update_Oneharder if length changes, preserve sequence

Retrieve_Fewrecords matching a set of criteria


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Unix Files

Unix files are simply streams of bytes and do not haveinternal structure as far as the operating system is

concerned.

Some OSs such as mainframe systems provide various file

structures the application can use.


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File Management System

Set of system software providing file services to users andapplications.

Users and applications access files through the file

management system.

Programmer does not need to develop file managementsoftware for each application.


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Objectives for a

File Management System

Meet the data management needs and requirements of the

user (described on next slide).

Guarantee that the data in the file are valid.

Optimize performance. Provide I/O support for a variety of storage device types.

Minimize or eliminate the potential for lost or destroyed

data.

Provide a standardized set of I/O interface routines Provide I/O support for multiple users.


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Meeting User Requirements

(the first objective listed on previous slide)

Each user:

Should be able to create, delete, read, and change files.

May have controlled access to other users files.

May control what type of accesses are allowed to the

users files.

Should be able to restructure the users files in a form

appropriate to the problem.

Should be able to move data between files.

Should be able to back up and recover the users files in

case of damage.

Should be able to access the users files by using symbolic

names.


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File System Architecture

The next slide illustrates a file system architecture.

The pieces involved are:

Device driver: communicates directly with hardware

device.

Basic file system: handles physical I/O.

Basic I/O supervisor: responsible for file I/O initiation and

termination, device selection, I/O scheduling, buffer

allocation.

Logical I/O: Enables users and applications to access

records. Provides record-oriented I/O capability.

Access method: access files based on their structure and

method of access.


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File Management Functions

See illustration on next slide: Identify and locate a selected file.

Use a directory to describe the location of all files plustheir attributes.

Enforce user access control (shared system). Employ access method on the records.

Provide blocking for file I/O.

Block for output and unblock for input.

Manage secondary storage Allocate files to available blocks.

Manage free storage of available blocks.

Scheduling of I/O requests


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File Organization and Access

Criteria for choosing a file organization:

Rapid Access

Ease of update

Economy of storage

Simple maintenance

Reliability

There may be trade-offs between criteria. Priority of criteria depends on how application will use the

file.


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File Organization and Access

There are a large number of file organizations.

Most actual file organizations are related to one of these

five fundamental file organizations:

Pile

Sequential file

Indexed sequential file

Indexed file

Direct or hashed file


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Pile

Data are collected in the order they arrive

Purpose is to accumulate a mass of data and save it

Records may have different structures or no structure.

Data access is by exhaustive search

May be useful to store data prior to processing


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Sequential File

Fixed format used for records.

Records are the same length.

All fields the same (order and length).

Field names and lengths are attributes of the file.

One field is the key field

Uniquely identifies the record

Records are stored in key sequence

New records are placed in a log file or transaction file. Log file is periodically merged with the master file.

Useful in batch processing.


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Indexed Sequential File

File is like a sequential file with key and data.

Index provides a lookup capability to quickly reach the

vicinity of the desired record.

Greatly reduces the time required to access a single record,

while still providing sequential processing.

Additions can be handled by an overflow file, which can

be periodically merged with the main file.


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Indexed File

Uses multiple indexes for different key fields.

Abandons sequential format with single key.

Records are accessed only by their index.

Records can be variable length.

Exhaustive index has pointers to every record.

Partial index only has pointers to records having the field

of interest.

Useful in applications such as an airline reservation system

that need to access particular records quickly, but rarely

need to process them sequentially.


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Direct or Hash File

Turn key value into an address to support direct access ofrecords in the file.

Gives very rapid access to data.

Useful if records are only accessed one at a time.


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B-Trees

Indexes are often implemented as B-trees.

The B-tree will have a large branching factor resulting in a

tree of low height.

This results in fewer disk accesses to traverse the tree in a

search.

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Directories

The directory contains information about the files,including attributes, location, and ownership.

The directory is itself a file, owned by the OS and

managed by the file management system.

The next slides give information typically found in thedirectory for each file.


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Directory Operations

Search: find a file in the directory.

Create file: add entry to directory when a file is created.

Delete file: remove entry from directory when file is

deleted.

List directory: all or a portion may be listed.

Update directory: change file attributes stored in directory.


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Directory Structure

A tree-structure is used for storing the file directory.

At the top is the master directory, beneath it are each user

directory.

Every level may contain files and subdirectories.


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Directory Naming

Users refer to files by their symbolic names.

Each name must be unique.

In a tree structure, the path to the file helps eliminate

duplicate name problems.

Most systems provide for a working directory, where file

names can be specified relative to it.

Users typically have a default, or home, directory upon

logging in.


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File Sharing

In a multiuser system, files should be able to be sharedbetween users.

This presents two issues:

Access rights

Simultaneous access


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File SharingAccess Rights

When sharing files, we have to have a way of controllingaccess to the file.

Typically one user is the owner.

The owner has all of the access rights and may grant rights

to others.


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Access rights may be given to different classes of users.

Typical classes are:

Specific user

Group of users

All:


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Examples of access rights: None: file is unknown to user.

Knowledge: user can determine file existence and owner.

Execution: can execute the file.

Reading: can read the file. Appending: can only append to end of file.

Updating: can modify file contents.

Changing protection: can change access rights.

Deletion: can remove the file.

These rights may constitute a hierarchy, with higher levelrights including the lower level ones.


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File Sharing - Simultaneous Access

When access is granted to append or update a file to morethan one user, the OS or file management system must

enforce discipline.

One approach is to lock the entire file when it is to be

updated. Another approach is to lock individual records during

update.


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Record Blocking

Users work with a logical view of a file as a set of records. The I/O device works with the physical view of a file as a

set of blocks.

Blocking of records is performed when writing data,

unblocking is performed when reading data.


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Record Blocking

Blocking considerations: The larger the block, the more records that are passed in a

single I/O.

For sequential access this reduces I/Os, but may result in

unnecessary data transfer for random access. Typically blocks are of fixed length to simplify the I/O

transfer.


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Record Blocking

There are three blocking methods: Fixed: fixed-length records. May cause internal

fragmentation.

Variable-length spanned: variable-length records

which may span blocks. Permits unlimited record size.May require multiple I/Os.

Variable-length unspanned: variable-length records

which do not span blocks. May cause internal

fragmentation.


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File Allocation

File allocation issues: Whether to allocate the maximum space required all at

once, or to allocate space in portions.

If portions are used, how big should a portion be?

If portions are used, how to keep track of the portions.


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Preallocation versus Dynamic Allocation

Preallocation requires that the maximum file size bedeclared and allocated when the file is created.

May create waste as users over-estimate required size to

ensure space needs are met.

Dynamic allocation allows portions of space to be given tothe file so that it may grow dynamically.


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Allocation Portion Size

Two basic methods: Large, variable-length contiguous portion.

Good performance.

May cause fragmentation.

Allocation table is simple, just need pointer to

beginning and length.

Small fixed-sized blocks.

Good flexibility.

Requires more complicated allocation table to keep

track of non-contiguous blocks.


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File Allocation Methods

Three methods of file allocation: Contiguous allocationa single contiguous set of blocks is

allocated to a file at the time of file creation. Thus, this is

a preallocation method of a variable-size portion. May

require periodic compaction due to fragmentation. Goodfor sequential file access.

Chained allocationallocate individual blocks. Each

block contains a pointer to the next block.

Indexed allocationan index specifies the location of eachblock.


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Free Space Management

It is necessary to know what space is available in order toperform allocations.

A disk allocation table keeps track of available space.

Techniques:

Bit tables: each bit represents a block

Chained free portions: free portions are chained

together.

Indexing: index keeps track of every free portion as if it

was a file.

Free block list: blocks are numbered and list of free

block numbers is maintained.


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based on Stalling's official slides 54

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