Chapter 12

File Management Systems

Chapter goals Describe the components and functions

of a file management system Compare the logical and physical

organization of files and directories Explain how secondary storage

locations are allocated to files and describe the data structures used to record those allocations

Chapter goals cont. Describe file manipulation operations,

including open, close, read, delete, and undelete operations

List access controls that can be applied to files and directories

Describe security, backup, recovery, and fault tolerance methods and procedures

File Management Systems FMS is implemented in layers like

the OS Command layer or application

program File control Storage I/O control Storage devices

File Man. Systems Layers

FMS Layers Storage devices – corresponds with

hardware layer of OS Interacts with the bus and with

operating system device drivers to transfer data between storage devices and memory

FMS layers Storage I/O control – accesses storage

locations and manages data movement between storage devices and memory

Part of OS kernel Its software modules include

Device drivers for each storage device or device controller

Interrupt handlers Buffers and cache managers

FMS layers cont. File Control – provides a set of service functions for

manipulating files and directories Processes service calls from users and applications Maintains directory and storage allocation data

structures used to locate files and their associated physical locations

Command layer or application program – users perform common file management functions such as copying, moving and renaming

Logical vs. physical view Logical Storage Views – viewed by

users are a collection of files organized within directories and storage volumes

Physical Storage Views – a collection of physical storage locations organized as a linear address space

Logical vs. physical view

Logical vs. physical view of a file File is subdivided into records Record usually contains

information about a single customer, thing such as a product in inventory, or an event

Records are divided into fields Fields are individual units of data

Logical vs. physical Logical file structure is independent

of its physical implementation Logical file structure “ignores”

Physical storage allocations – records can be stored in separate file locations

Data access methods Data encoding methods

Logical structure of data file

File content and type A file is a collection of data created by an

applications The format of that information is called

the file type A file can store many different data types

including text, numbers, complex data structures, and executable instructions

Modern file management systems provide a framework to support additional file types

File type File type normally is declared when

a file is created In the UNIX file management

system, the file type is stored within the directory. In the Windows file management system, the file type is declared through the extension

Registered Windows File Types In windows the extension of a file can

be associated (connected) to a specific application

When you open a file associated with an application, that application is started and the file is opened in the associated application

Show associated file types – control panel, folder options

Directory content and structure Files are organized into directories Some directories are created and

maintained by software Windows directories Directories associated with

applications User can also create and maintain

directories

Directory content Directories in windows are organized in

a hierarchy Directory information includes:

Name File type Location Size Ownership Access controls Time stamp(s)

Show directory information for C drive

Directory Content and Structure Typical file ownership permissions are:

Create Read Update delete

Time stamps include: When the file was created When the file most recently was read When the file most recently was written When the file last was backed up

Hierarchical Directory Structure Windows directory structure is

hierarchical Directories can contain other

directories (called sub-directories) Directories can not have more

than one parent Sometimes called a tree structure

(draw picture)

Hierarchical directory structures

Storage allocation Users and applications programs

continually create and change files When files are created they must

be given space in storage When files grow they must be

given additional space When files shrink unneeded space

must be released

Allocation units Allocation unit is smallest unit of

space that can be allocated to a file Allocation units cannot be smaller

than system data transfer unit Data transfer unit is called a block Block sizes range from 512 bytes to

4 KB in multiples of 512 bytes

Allocation units cont. Is a multiple of block size, i.e.

equal to 4 blocks, 8 blocks, etc. Size of allocation unit is optimized

to use space efficiently

Storage allocation tables FMS maintains a table of storage

allocation units Records which units belong to

which file Records which units are available

(free) In windows this table called File

Allocation Table or FAT

Sample allocations of files The next three slides show 3 files,

how their storage is allocated, and how FMS records location of allocation units

Storage units are said to be “chained together” using pointer

Each unit contains reference (pointer) to next unit in the list

Directory listing of 3 files

Where 3 files are stored

How FMS finds files

Blocking and buffering A logical record is a collection of

data items, or fields, that is accessed by an application program as a single unit

A physical record is the unit of storage transferred between the device controller and memory in a single operation

Blocking and buffering Most of the time the logical size of a

record does not equal the physical allocation unit

When several logical records are grouped

within physical records that is called blocking

Individual logical records grouped together into one unit called a block

Blocking and buffering cont. Simplifies data transfer between

drive and memory Then OS must extract individual

records from the allocation unit block

If a physical record contains just one logical record, then the file is said to be unblocked

Blocking records

Buffer Buffer is storage area in memory

where blocks of records from drive are copies

Buffer is used to extract individual records from a block

Using buffers

Operations on files There are setup requirements that

need to be executed whenever a file is first requested by an application and when that file is not longer needed

These operations are called File open File close

File open When an application requests a

file, the FMS must do the follow: The file must be located on the

storage device Ensure that application has right to

access this file Allocates one or more buffers Updates internal table of open files

File close When application sends request to

close a file, FMS does the following: Flushing the program’s I/O buffers to

secondary storage De-allocate buffer memory Update directory entry time stamp Update file open table

Delete and Undelete Operations In most file management systems, files

are not removed immediately from secondary storage when they are deleted

The file’s storage allocation units are marked as free and its directory entry is marked as unused

A user might be able to use the undelete operation to recover the file

Recyling bin in windows allows files to be undeleted

Access controls A File Management System helps

prevent loss, corruption and unauthorized access to files

The operating system is used to identify and authenticate users and their processes

The file access is authenticated through id’s and passwords

Access control For Example:

UNIX defines three access control types:

Read Write Execute

Internet web sites – you can read pages but not change them (unless you are a hacker)

FMS Backup options Full Backup – the FMS copies all files

and directories for an entire storage volume

Incremental Backup – only the files that have been modified are archived

Differential Backup – only the changed portions of the files are archived

Windows backup Programs accessories system tools

Summary The file management system (FMS), usually

a part of the operating system, manages all aspects of user and program access to secondary storage

With directories, users can organize the thousands of files stored in a typical computer system

Secondary storage units are divided into allocation units, which are typically a few kilobytes in size

Summary cont. The FMS allocates buffers to support

program file I/O The FMS enforces access controls when

accessing files on behalf of a user or program FMSs provide utilities to make backup copies

of files and directories and to recover them if needed