Understanding Operating Systems Sixth Edition Chapter 8 File Management.

Understanding Operating Systems Sixth Edition

Chapter 8File Management

Understanding Operating Systems, Sixth Edition

Learning Objectives

After completing this chapter, you should be able to describe:

• The fundamentals of file management and the structure of the file management system

• File-naming conventions, including the role of extensions

• The difference between fixed-length and variable-length record format

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Learning Objectives (cont'd.)

• The advantages and disadvantages of contiguous, noncontiguous, and indexed file storage techniques

• Comparisons of sequential and direct file access

• Access control techniques and how they compare

• The role of data compression in file storage

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The File Manager• The File Manager controls every file in the system.

• The File Manager (File Management System) is the software responsible for creating, deleting, modifying, and controlling access to files, as well as for managing the resources used by the files.

• Provides support for:– Libraries of programs and data to online users; – Spooling operations;– Interactive computing.

• These functions are performed in collaboration with the Device Manager.

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Responsibilities of the File Manager• The File Manager is in charge of the system’s

physical components, its information resources, and the policies used to store and distribute the files.

• To carry out its responsibilities, it must perform four tasks:– Keep track of where each file is stored.– Use a policy that will determine:

• Where and how the files will be stored;• Efficiently use the available storage space;• Provide efficient file access to the files.

– Allocate each file when a user has been cleared for access to it;

• Record its use.

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Responsibilities of the File Manager (cont’d)

– Deallocate the file when the file is to be returned to storage;

– Communicate the file availability to others who may be waiting for it.

• The File Manager keeps track of its files with directories that contain:– the filename;– its physical location in secondary storage;– Other important information about each file.

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Responsibilities of the File Manager (cont'd.)

• The File Manager’s policy determines:– Where each file is stored;– How the system, and its users, will be able to access

them.• Via device-independent commands.

– The policy must determine who will have access to what material.

• This involves two factors:– Flexibility of access to the information;– Its subsequent protection.

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• The File Manager does this by:– Allowing access to shared files;– Providing distributed access;– Allowing users to browse through public directories.

• The OS must:– Protect its files against system malfunctions;– Provide security checks via account numbers and

passwords to preserve the integrity of the data;– Safeguard against tampering.

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• The computer system allocates a file by activating the appropriate secondary storage device and loading the file into memory while updating its records of who is using what file.

• The File Manager deallocates a file by updating the file tables and rewriting the file (if revised) to the secondary storage device.

• Any processes waiting to access the file are then notified of its availability.

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Responsibilities of the File Manager Definitions

• Field– A group of related bytes that can be identified by the

user with a name, type, and size.• Record

– A group of related fields.• File

– A group of related records that contains information to be used by specific application programs to generate reports.

– Flat file• No connections to other files, no dimensionality

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• Database– Groups of related files that are interconnected at

various levels to give users flexibility of access to the stored data.

– If the user’s database requires a specific structure, the File Manager must be able to support it.

• Program files– Contain instructions; – Data files contain data;– As far as storage is concerned, the File Manager

treats them the same way.

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• Directories– Special files with listings of filenames and their

attributes. – Data collected to monitor system performance and

provide for system accounting is collected into files.– Every program and data file accessed by the

computer system, as well as every piece of computer software, is treated as a file.

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Interacting with the File Manager• The user communicates with the File Manager,

which responds to specific commands.– OPEN, DELETE, RENAME, COPY

• Files can also be created with other system-specific terms:– The first time a user gives the command to save a

file, it’s actually the CREATE command.– In other OS, the OPEN NEW command within a

program indicates to the File Manager that a file must be created.

• These commands and many more were designed to be very simple to use.

• They are device independent.

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Interacting with the File Manager (cont’d)

• Device Independent– To access a file, the user doesn’t need to know its

exact physical location on the disk pack• Cylinder• Surface• Sector

– The medium in which it’s stored• Archival tape• Magnetic disk• Optical disc• Flash Storage

– The network specifics.

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Interacting with the File Manager (cont'd.)

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• File access is a complex process.• Each logical command is broken down into a

sequence of signals that trigger the step-by-step actions performed by the device and supervise the progress of the operation by testing the device’s status.

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• For example, when a user’s program issues a command to read a record from a disk, the READ instruction has to be decomposed into the following steps:– Move the read/write heads to the cylinder or track

where the record is to be found.– Wait for the rotational delay until the sector containing

the desired record passes under the read/write head.– Activate the appropriate read/write head and read the

record.– Transfer the record to main memory.– Send a flag to indicate that the device is free to satisfy

another request.

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• While all of this is going on, the system must check for possible error conditions.

• The File Manager does all of this, freeing the user from including in each program the low-level instructions for every device to be used.

• Without the File Manager, every program would need to include instructions to operate all of the different types of devices and every model within each type.

• It would be impractical to require each program to include these minute operational details.– The advantage of device independence.

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Interacting with the File Manager Typical Volume Configuration

• Normally the active files for a computer system reside on secondary storage:– CDs– DVDs– Floppy disks– USB devices– Hard disks– Nonremovable disk packs – Other removable media

• Files that aren’t frequently used can be stored offline and mounted only when the user specifically requests them.

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• Each storage unit (removable or not) is considered a volume.

• Each volume can contain several files.– Multifile volumes.

• Some files are extremely large and are contained in several volumes.– Multivolume files.

• Each volume in the system is given a name.• The File Manager writes this name and other

descriptive information on an easy-to-access place on each unit.

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– The innermost part of the CD or DVD;– The beginning of the tape;– The first sector of the outermost track of the disk pack

• Once identified, the OS can interact with the storage unit.

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• Master file directory (MFD) – Is stored immediately after the volume descriptor.– Lists the names and characteristics of every file

contained in that volume.– The filenames in the MFD can refer to:

• Program files;

• Data files;

• System files;

• Subdirectories;– If the File Manager supports subdirectories.

– The remainder of the volume is used for file storage.

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• Master File Directory (MFD)– The first OSs supported only a single directory per volume.

• This directory was created by the File Manager and contained the names of files, usually organized in alphabetical, spatial, or chronological order.

– Major Disadvantages:• It would take a long time to search for an individual file.• If the user had more than 256 small files stored in the

volume, the directory space (with a 256 filename limit) would fill before the disk storage space filled up. The user would then receive a message of “disk full” when only the directory itself was full.

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• Master File Directory (MFD) (cont’d)– Major Disadvantages (contid):

• Users couldn’t create subdirectories to group the files that were related.

• Multiple users couldn’t safeguard their files from other users because the entire directory was freely made available to every user in the group on request.

• Each program in the entire directory needed a unique name, even those directories serving many users.

– Only one person using that directory could have a program named Program1.

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Interacting with the File Manager Introducing Subdirectories

• File Managers create an MFD for each volume that can contain entries for both files and subdirectories.

• A Subdirectory is created when a user opens an account to access the computer system.

• Although the user directory is treated as a file, its entry in the MFD is flagged to indicate to the File Manager that this file is really a subdirectory and has unique properties:– Its records are filenames pointing to files.

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Interacting with the File Manager Introducing Subdirectories (cont’d)

• Although this was an improvement from the single directory scheme, it didn’t solve the problems encountered by prolific users who wanted to group their files in a logical order to improve the accessibility and efficiency of the system.

• Today’s File Managers encourage users to create their own subdirectories, so related files can be grouped together.– Folders

• This structure is an extension of the previous two-level directory organization, and it’s implemented as an upside-down tree (Figure 8.4)

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Interacting with the File Manager Introducing Subdirectories (cont'd.)

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• Tree structures allow the system to efficiently search individual directories because there are fewer entries in each directory.– However, the path to the requested file may lead

through several directories.• For every file request, the MFD is the point of entry.• It’s accessible only by the OS.• When the user wants to access a specific file:

– The filename is sent to the File Manager.– The File Manager first searches the MFD for the

user’s directory.– It then searches the user’s directory and any

subdirectories for the requested file and its location.30



• Each file entry in every directory contains information describing the file.

• File Descriptor:– Filename – Within a single directory, filenames must be

unique; in some OS, the filenames are case sensitive.– File type – The organization and usage that are

dependent on the system (files and directories).– File size – Although it could be computed from other

information, the size is kept here for convenience.– File location – Identification of the first physical block

(or all blocks) where the file is stored. – Date and time of creation

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• File Descriptor:– Owner– Protection information - Access restrictions, based on

who is allowed to access the file and what type of access is allowed.

– Record size – Its fixed size or its maximum size, depending on the type of record.

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Interacting with the File Manager File-Naming Conventions

• A file’s name can be much longer than it appears.• Depending on the File Manager, it can have from

two to many components.• The two components common to many filenames

are:– Relative filename– An extension

• The relative filename is the name that differentiates it from other files in the same directory.– Can vary in length from one to many characters and

can include letters of the alphabet, as well as digits.

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Interacting with the File Manager File-Naming Conventions (cont’d)

• Relative Filename– Every OS has specific rules that affect the length of

the relative name and the types of characters allowed.– Most OS allow names with dozens of characters

including spaces, hyphens, underlines, and certain other keyboard characters.

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Interacting with the File Manager File-Naming Conventions (cont'd.)

• Extensions– Some OS require an extension that’s appended to the

relative filename.– It’s usually two or three characters long and is

separated from the relative name by a period.– Its purpose is to identify the type of file or its contents.– Example

• BASIA_TUNE.MP3

• TAKE OUT MENU.RTF

• TAKE OUT MENU.DOC

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• Extensions– If an extension is incorrect or unknown

• Requires user intervention (Figure 8.5).

– Some extensions (EXE, BAT, COB, FOR) are restricted by certain OS because they serve as a signal to the system to use a specific compiler or program to run these files.

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• There may be other components required for a file’s complete name.– These other components are OS specific.

• Depending on the OS, there may be other components required for a file’s complete name:

INVENTORY_COST.DOC

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• INVENTORY_COST.DOC– Windows:

• The file’s complete name is composed of its relative name and extension, preceded by the drive label and directory name:

– C:\IMFST\FLYNN\INVENTORY_COST.DOC

» This indicates to the system that the file INVENTORY_COST.DOC requires a word processing application program, and it can be found in the directory IMFST; subdirectory FLYNN in the volume residing on drive C.

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• INVENTORY_COST.DOC– UNIX/Linux

/usr/imfst/flynn/inventory_cost.doc• The first entry is represented by the forward slash (/).

– This represents a special master directory called the root.

• Next is the name of the first subdirectory: usr/imfst

• Followed by a sub-subdirectory: /flynn

• The final entry is the file’s relative name: inventory_cost.doc

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• The names tend to grow in length as the File Manager grows in flexibility.

• The folders on a system with a GUI (Windows or Macintosh) are actually directories or subdirectories.– When someone creates a folder, the system creates

a subdirectory in the current directory or folder.

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File Organization

• The arrangement of records within a file.– All files are composed of records.

• When a user gives a command to modify the contents of a file, it’s actually a command to access records within the file.

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File OrganizationRecord Format

• All files are composed of records.• Within each file, the records are all presumed to

have the same format:– Fixed length– Variable length

• The records, regardless of their format can be blocked or unblocked.

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• Fixed-Length Records– Are the most common because they’re the easiest to

access directly.• Ideal for data files.

– The critical aspect of fixed-length records is the size of the record.

• If the record is too small, smaller than the number of characters to be stored in the record, the leftover characters are truncated.

• If the record is too large, larger than the number of characters to be stored, storage space is wasted.

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• Variable-Length Records– Don’t leave empty storage space and don’t truncate

any characters• Eliminates the two disadvantages of fixed-length

records.– While they can be easily read (one after the other),

they’re difficult to access directly because it’s hard to calculate exactly where the record is located.

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• Variable-Length Records– Used most frequently in files that are likely to be

accessed sequentially• Text files• Program files• Files that use an index to access their records.

– The record format, how it’s blocked, and other related information is kept in the file descriptor.

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File OrganizationPhysical File Organization

• Describes the way records are arranged and the characteristics of the medium used to store it.

• On magnetic disks (hard drives), files can be organized in one of several ways:– Sequential– Direct– Indexed sequential.

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• To select the best of these file organizations, the programmer considers these practical characteristics: – Volatility of data

• The frequency with which additions and deletions are made.

– Activity of the file• The percentage of records processed during a given

run.

– Size of the file

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• Practical Characteristics (cont’d): – Response time

• The amount of time the user is willing to wait before the requested operation is complete.

– Especially crucial when doing time-sensitive searches.

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• Sequential Record Organization– The easiest to implement because records are stored

and retrieved serially, one after the other.– To find a specific record, the file is searched from its

beginning until the requested record is found.

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Physical File Organization (cont’d)• Sequential Record Organization (cont’d)

– To speed the process, some optimization features may be built into the system.

• Select a key field from the record and then sort the records by that field before storing them.

• Later, when a user requests a specific record:– The system searches only the key field of each record in

the file.– The search is ended when either an exact match is

found or the key field for the requested record is smaller than the value of the record last compared, in which case the message “record not found” is sent to the user and the search is terminated.

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Physical File Organization (cont’d)• Sequential Record Organization (cont’d)

– Although this technique aids the search process, it complicates file maintenance because the original order must be preserved every time records are added or deleted.

– To preserve the physical order, the file must be completely rewritten or maintained in a sorted fashion every time it’s updated.

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Physical File Organization (cont'd.)• Direct Record Organization:

– Uses direct access files, which can be implemented only on direct access storage devices.

– These files give users the flexibility of accessing any record in any order without having to begin a search from the beginning of the file to do so.

• Random organization

• Random access files

– Records are identified by their relative addresses • Their addresses relative to the beginning of the file

– These logical addresses are computed when the records are stored, and then again when the records are retrieved.

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– The user identifies a field (or combination of fields) in the record format and designates it as the key field because it uniquely identifies each record.

– The program used to store the data follows a set of instructions, called a hashing algorithm, that transforms each key into a number – the record’s logical address.

– This is given to the Filer Manager, which takes the necessary steps to translate the logical address into a physical address (cylinder, surface, and record numbers), preserving the file organization.

– The same procedure is used to retrieve a record.55



– A direct access file can also be accessed sequentially, by starting at the first relative address and going to each record down the line.

– Direct access files can be updated more quickly than sequential files because the records can be quickly rewritten to their original addresses after modifications have been made.

• There’s no need to preserve the order of the records so adding or deleting them takes very little time.

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– The problem with hashing algorithms is that several records with unique keys may generate the same logical address and then there’s a collision.

– When that happens, the program must generate another logical address before presenting it to the File Manager for storage.

– Records that collide are stored in an overflow area that was set aside when the file was created.

– Although the program does all the work of linking the records from the overflow area to their corresponding logical address, the File Manager must handle the physical allocation of space.

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Physical File Organization (cont'd.)

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– The maximum size of the file is established when it’s created, and eventually either the file might become completely full or the number of records stored in the overflow might become so large that the efficiency of retrieval is lost.

• At that time, the file must be reorganized and rewritten, which requires intervention by the File Manager.

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Physical File Organization (cont'd.)• Indexed Sequential Record Organization

– Combines the best of sequential and direct access.– It’s created and maintained through an Indexed

Sequential Access Method (ISAM) application.– Removes the burden of handling overflows and

removes record order from the shoulders of the programmer.

– Doesn’t create collisions because it doesn’t use the results of the hashing algorithm to generate a record’s address.

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– It generates an index file through which the records are retrieved.

– This organization divides an ordered sequential file into blocks of equal size.

• Their size is determined by the File Manager to take advantage of physical storage devices and to optimize retrieval strategies.

– Each entry in the index file contains the highest record key and the physical location of the data block where this record, and the records with smaller keys are stored.

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– To access any record in the file, the system begins by searching the index file and then goes to the physical location indicated by that entry.

– The index file acts as a pointer to the data file.– An indexed sequential file also has overflow areas

spread throughout the file so expansion of existing records can take place and new records can be located in close physical sequence as well as in logical sequence.

– Another overflow area is located apart from the main data area but is used only when the other overflow areas are completely filled.

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– When retrieval time becomes too slow, the file has to be reorganized.

• Usually performed b y maintenance software.

• For most dynamic files, indexed sequential is the organization of choice because it allows both direct access to a few requested records and sequential access to many.

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Physical Storage Allocation • The File Manager must work with files not just as whole

units but also as logical units.• Records within a file must have the same format but they

can vary in length.• Records are subdivided into fields.• Their structure is managed by application programs and

not the OS.– An exception is made for those systems that are heavily

oriented to database applications, where the File Manager handles field structure.

• When we talk about file storage, we are actually referring to record storage.

• The File Mgr and the Device Mgr have to cooperate to ensure successfully record storage and retrieval.

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Physical Storage Allocation (cont'd.)

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Physical Storage Allocation Contiguous Storage

• Records stored one after another.

• Used in early OS– Advantages:

• Any record can be found and read once its starting address, size are known (Streamlined Directory)

• Ease of direct access because every part of the file is stored in the same compact area.

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– Disadvantages:• A file can’t be expanded un less there’s empty space

available immediately following it (Figure 8.9).– Room for expansion must be provided when the file is

created.– If these’s not enough room, the entire file must be

recopied to a larger section of the disk every time records are added.

• Fragmentation– Can be overcome by compacting and rearranging files.– Files can’t be accessed while compaction is taking

place.

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– The File Manager keeps track of the empty storage areas by treating them as files.

• They’re entered in the directory but are flagged to differentiate them from real files.

– Usually the directory is kept in order by sector number, so adjacent empty areas can be combined into one large free space.

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Physical Storage Allocation Noncontiguous Storage

• Allows files to use any storage space available on the disk.

• A File’s records are stored in a contiguous manner, only if there’s enough empty space.

• Any remaining records and all other additions to the file are stored in other sections of the disk.– Extents of the file

• are linked together with pointers;

• The physical size of each extent is determined by the OS:

• Usually 256 bytes– Or another power of two – bytes.

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• File extents are usually linked in one of two ways:– Storage Level

• Each extent points to the next one in the sequence (Figure 8.10).

• The Directory entry consists of the:– Filename– Storage location of the first extent– Location of the last extent– Total number of extents (not counting first).

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• File extents are usually linked in one of two ways (cont’d):– Directory Level

• Each extent is listed with its physical address, its size, and a pointer to the next extent.

• A null pointer indicates that it’s the last one (Shown in Figure 8.11 as a hyphen (-)).

• Although both noncontiguous allocation schemes eliminate external storage fragmentation and the need for compaction, they don’t support direct access because there’s no easy way to determine the exact location of a specific record.

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• Files are usually declared to be either sequential or direct when they’re created.– so the File Manager can select the most efficient

method of storage allocation:• Contiguous for direct files;

• Noncontiguous for sequential.

– OSs must have the capability to support both storage allocation schemes.

– Eliminates external storage fragmentation– Eliminates need for compaction

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• Files are usually declared to be either sequential or direct when they’re created.– Disadvantage

• No direct access support– Cannot determine specific record’s exact location

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Physical Storage Allocation Indexed Storage

• Allows direct record access by bringing together the pointers linking every extent of that file into an index block.

• Every file has its own index block, which consists of: – The addresses of each disk sector that make up the

file.

• The index lists each entry in the same order in which the sectors are linked (Figure 8.12).– The third entry in the index block corresponds to the

third sector making up the file.76

Understanding Operating Systems, Sixth Edition 77




– When a file is created, the pointers in the index block are all set to null.

– As each sector is filled, the pointer is set to the appropriate sector address.

• The address is removed from the empty space list and copied into its position in the index block.

• The scheme supports both sequential and direct access.– It doesn’t necessarily improve the use of storage

space because each file must have an index block• Usually the size of one disk sector.

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– For larger files with more entries, several levels of indexes can be generated in which case, to find a desired record:

• The File Manager accesses the first index;– The highest level.– Which points to a second index (lower level)– Which points to an even lower-level index– Which eventually points to the data record.

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Access Methods• Dictated by a file’s organization.

– The most flexibility is allowed with indexed sequential files;

– The least flexibility is with sequential files.• A file that has been organized in sequential fashion

can support only sequential access to its records.• These records can be of either fixed or variable length.

• The File Manager uses the address of the last byte read to access the next sequential record.– The Current byte address (CBA) must be updated

every time a record is accessed.• Such as when a READ command is executed.

• Figure 8.13 shows the difference between storage of fixed-length and of variable-length records.

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Access Methods (cont'd.)

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Access MethodsSequential Access

• Sequential Access of Fixed-Length Records:– The CBA is updated simply by incrementing it by the

record length (RL), which is constant.• CBA = CBA + RL

• Sequential Access of Variable-length records:– The File Manager adds the length of the record (RL)

plus the numbers of bytes used to hold the record length (N, which holds the constant shown as m, n, p, or q, in Figure 8.13) to the CBA,

• CBA = CBA + N + RL

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Access Methods Direct Access

• If a file is organized in direct fashion, it can be accessed in either direct or sequential order if the records are of fixed length.

• In the case of direct access with fixed-length records:– The CBA can be computed directly from the record

length and the desired record number (information provided through the READ command) minus 1.

• CBA = (RN – 1) * RL– If we’re looking for the beginning of the eleventh record

and the fixed record length is 25 bytes, the CBA would be:

» (11-1) * 25 = 250

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• If the file is organized for direct access with variable-length records:– It’s virtually impossible to access a record directly

because the address of the desired record cannot be easily computed.

• To access a record:– The File Manager must do a sequential search

through the records.– It becomes a half-sequential read through the file

because the File Manager could save the address of the last record accessed.

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• To access a record (cont’d):– When the next request arrives, it could search

forward from the CBA if the address of the desired record was between the CBA and the end of the file.

– Otherwise, the search would start from the beginning of the file.

• An alternative is for the File Manager to keep a table of record numbers and their CBAs.– To fill a request:

• The table is searched for the exact storage location of the desired record.

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• Records in an Indexed sequential file can be accessed either sequentially or directly.– Either of the procedures to compute the CBA presented

would apply with one extra step:• The Index file must be searched for the pointer to the

block where the data is stored.

– Because the index file is smaller than the data file, it can be kept in main memory, and a quick search can be performed to locate the block where the desired record is located.

– The block can then be retrieved from secondary storage, and the beginning byte address of the record can be calculated.

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Levels in a File Management System

• The efficient management of files can’t be separated from the efficient management of the devices that house them.

• The highest level module is called the “basic file system”.– It passes information through the access control

verification module to the logical file system.• Which notifies the physical file system;

• Which works with the Device Manager.

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• Each level of the file management system is implemented using structured and modular programming techniques that also set up a hierarchy.

• The higher positioned modules pass information to the lower modules.

• They, in turn, can perform the required service and continue the communication down the chain to the lowest module.

• Which communicates with the physical device and interacts with the Device Manager.

• Only then is the record made available to the user.

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• Each of the modules can be further subdivided into more specific tasks:

READ RECORD NUMBER 7 FROM FILE CLASSES INTO STUDENT

• CLASSES is the name of a direct access file previously opened for input.

• STUDENT is a data record previously defined within the program and occupying specific memory locations.

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• Because the file has already been opened, the file directory has already been searched to verify the existence of CLASSES, and pertinent information about the file has been brought into the OS’s active file table.

• This information includes:– Record Size;– The address of its first physical record;– Its protection;– Access control information.

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• This information is used by the basic file system, which activates the access control verification module to verify that this user is permitted to perform this operation with this file.– If access is allowed, information and control are

passed along to the logical file system.– If access is not allowed, a message saying “access

denied” is sent to the user.

• The logical file system transforms the record number to its byte address: CBA = (RN-1) * RL

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• This result, together with the address of the first physical record and, in the case where records are blocked, the physical block size, is passed down to the physical file system, which computes the location where the desired record physically resides.

byte addressBlock number = integers [-----------------------] + address of the first physical record

• physical block size

byte addressoffset = remainder [-----------------------]

physical block size

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• This information is passed on to the device interface module which transforms the block number to the actual cylinder/surface/record combination needed to retrieve the information from the secondary storage device.

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• Once retrieved, and, using the device-scheduling algorithms, the information is placed in a buffer and control returns to the physical file system, which copies the information into the desired memory location.

• When the operation is complete, the “all clear” message is passed on to all other modules.

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• A WRITE command is handled in exactly the same way until the process reaches the device handler.

• At that point, the portion of the device interface module that handles allocation of free space, the allocation module, is called into play because it’s responsible for keeping track of unused areas in each storage device.

• Verification, the process of making sure that a request is valid, occurs at every level of the file management system.

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• Verification process:– The first verification occurs at the directory level when

the file system checks to see if the requested file exists.– The second occurs when the access control verification

module determines whether access is allowed.– The third occurs when the logical file system checks to

see if the requested byte address is within the file’s limits.

– Finally, the device interface module checks to see whether the storage device exists.

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Access Control Verification Module• The first OS couldn’t support file sharing among

users.– Early systems needed 10 copies of a compiler to

serve 10 users.– Today’s systems require only a single copy to serve

everyone.• Any file can be shared

– Data files, user-owned program files, system files.• Advantages:

– It saves space;– Allows for the synchronization of data updates.

• As when two application program are updating the same data file.

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Access Control Verification Module

• Advantages (cont’d):– It improves the efficiency of the system’s resources.

• If files are shared in main memory, then there’s a reduction of I/O operations.

• Disadvantages:• The integrity of each file must be safeguarded.

– Calls for control over who is allowed to access the file– What type of access is permitted.

• Five possible actions that can be performed on a file:– READ only, WRITE only, EXECUTE only, DELETE

only;– Some combination of the four.

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Access Control Verification ModuleAccess Control Matrix

• Easy to implement.

• Because of it size it only works well for systems with a few files and a few users.

• In the matrix:– Each column identifies a user and each row identifies

a file.– The intersection of the row and column contains the

access rights for that user to that file.

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• In The Matrix:– Access rights identified by the letters RWED:

• R = Read

• W = Write

• E = Execute Address

• D = Delete Access

• (-) = Access not allowed

• Table 8.2 illustrates.

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• In the actual implementation, the letters, RWED are represented by bits 1 and 0;– 1 indicates that access is allowed;– 0 indicates that access is denied.

• As the numbers of files and users increase, the matrix becomes extremely large – sometimes too large to store in main memory.

• A lot of space is wasted because many of the entries are all null.

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Access Control Verification ModuleAccess Control Lists

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Access Control Verification ModuleAccess Control Lists

• Some systems shorten the access control list even more by putting every user into a category: – SYSTEM or ADMIN

• System personnel who have unlimited access to all files in the system.

– OWNER• Has absolute control over all files created in the owner’s

account.– GROUP

• All users belonging to the appropriate group have access.– WORLD

• All other users in system.

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Access Control Verification ModuleCapability Lists

• Lists every user and the files each has access to.

• In OS such as LINUX or UNIX, they can control access to devices as well as to files.

• The most commonly used is the access control list.

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Data Compression• Data Compression algorithms consist of two types:

– Lossless algorithms typically used for text or arithmetic files.

• Retains all the data in the file throughout the compression-decompression process.

– Lossy algorithms typically used for image and sound files.

• Remove data permanently.– Unwanted noise– Tones beyond a human’s ability to hear– Light spectrum that we can’t see

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Data CompressionText Compression

• Three methods to compress text in a database:– Records with repeated characters;– Repeated terms;– Front-end compression.

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• Records with repeated characters:– Data in a fixed-length field might include a short name

followed by many blank characters.– This can be replaced with a variable-length field and a

special code to indicate how many blanks were truncated.

ADAMSbbbbbbbbbb

Encoded looks like this:

ADAMSb10

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• Records with repeated characters:– Likewise, numbers with many zeroes can be

shortened, with a code (#) to indicate how many zeroes must be added to recreate the original number.

300000000

Encoded looks like this:

3#8

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• Repeated terms:– Compressed by using symbols to represent each of

the most commonly used words in the database.– University student database common words:

• Student, course, grade, department could each be represented with single character

– Of course, the system must be able to distinguish between compressed and uncompressed data.

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• Front-end compression:– Builds on the previous data element.

• The student database where the students’ names are kept in alphabetical order could be compressed (Table 8.6).

– Entry takes given number of characters from previous entry that they have in common.

– Storage space is gained, but processing time is lost.– The system must be able to distinguish between

compressed and uncompressed data.

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Data CompressionOther Compression Schemes

• Lossy compression allows a loss of data from the original file to allow significant compression.– The compression process is irreversible as the

original file cannot be reconstructed.

• The specifics of the compression algorithm are highly dependent on the type of file being compressed.– JPEG – a popular option for still images– MPEG – for video images

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Data CompressionOther Compression Schemes

• The International Organization for Standardization (ISO) has issued MPEG standards that “are international standards dealing with the compression, decompression, processing, and coded representation of moving pictures, audio, and their combination.”

• ISO is the world’s leading developer of international standards.

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Summary• The File manager controls every file in the system

and processes the user’s commands:– Read, write, modify, create, delete, etc.)– To interact with any file on the system.

• It also manages the access control procedures to maintain the integrity and security files under its control.

• The File Manager must be able to accommodate a variety of file organizations, physical storage, allocation schemes, record types, and access methods.

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Summary (cont’d)• In this chapter we discussed:

– Sequential, direct, and indexed sequential file organization;

– Contiguous, noncontiguous, and indexed file storage allocation;

– Fixed-length versus variable-length records;– Three methods of access control;– Data compression techniques.

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Summary (cont’d)• To get the most from a File Manager, it’s important

for users to realize:– The strengths and weaknesses of its segments;– Which access methods are allowed on which devices;– With which record structures;– The advantages and disadvantages of each in overall

efficiency.

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Understanding Operating Systems Sixth Edition Chapter 8 File Management.

Documents

file availability

file manager contd

file management slide

efficient file access

file managers policy

edition responsibilities

device manager

edition chapter