file management in general slides
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File Management
File Manager System• Software responsible
– for creating, deleting, modifying a physical files– Controlling access to files– Managing the resources used by files– Provides support
• for libraries of programs• Data to on-line users• Spooling operations• Interactive computing
• Functions are performed in collaboration with the Device manager
• Has a complex job. In charge of the system’s physical components, its information resource, and the policies used to store and distribute the files.
File ManagementTo carry out these responsibilities it must perform four tasks
1. Keep track of where each file is stored.
2. Use a policy that will determine where and how the files will be stored, making sure to efficiently use the available storage space and provide efficient access to the files.
3. Allocate each file when a user has been cleared for access to it, then record its use.
4. De allocate the file when the file is to be returned to storage, and communicate its availability to others who may be waiting for it.
Files System is like a LIBRARY• File manager playing the part of the librarian who performs the
same four tasks
1. A librarian uses the card catalog to keep track of each item in the collection: the cards list the call number and the details that help the patrons find each book.
2. The library relies on a predetermined policy to store everything in the collection including oversized book, magazines, recording, maps etc. And they must be arranged so people can find what they need.
3. When it is required, the book is retrieved from its shelf and the borrower’s name is recorded in the circulation file.
4. When the book is returned, the librarian deletes the entry from the circulation file and re-shelves the item.
Basic Definition relate to File Manager• Field – is a group of related bytes that can be identified by
the user with name, type, and size.• Record – is a group of related fields.• File – is a group of related records that contains
information to be used by specific application program.• Database – appears to the file manager to be a type
of file, but are more complex because they’re actually groups of related files that are interconnected
• Program files – contains instructions• Data files – contain data• Directories – are listings of file names and their
attributes
Example
Nama umur sekolah Penjaga
Mohd Syakirin 19 KMK Rusdi
Syahida Husna 17 SMSA Rusdi
Syahirah Husna 14 SMSB Rusdi
Syazwina Husna 12 SKTR Rusdi
Mohd. Syazwan 12 SKTR Rusdi
Syafiqah Husna 10 SKTR Rusdi
Field
recordfile
Interacting With File Manager
• User communicates with the file manager via a specific commands that may be either– Embedded in the user’s program
• E.q OPEN, CLOSE, READ, WRITE, MODIFY– Submitted interactively by the user.
• E.q CREATE, DELETE, RENAME, COPY• Design to be simple and device
independent
Typical Volume Configuration• Normally the active files for a computer system reside on
secondary storage units.– Removable storage units
• such as tapes, floppy disks and removable disk pack
• Files not frequently used can be stored offline and mounted only when the user specifically request
– Integrated storage unit such as drums, hard disks and non removable disk packs
• Both is considered a volume– ‘multi-file volume’ – a volume contain several files– ‘multi-volume file’ – extremely large file contained in
several volumes.
Volume Configuration - continue
• As an analogy–We have many college
accommodation –Each college we can consider as a
volume
Volume Configuration - continue• Each volume is given a name• The File Manager writes the name and other
descriptive information on an easy-to-access place – at the beginning of the magnetic tape or – first sector of the outermost track of the disk pack– Example of volume descriptor:-
Creation Date
Pointer to Directory Area
Pointer to File Area
File System Code
Volume Name
Date When volume was created
Indicates first sector where directory is stored.
Indicates first sector where file is stored
Used to detect volumes with incorrect formats
User-allocated Name
Volume Configuration - continue• As an analogy
– We have many college accommodation – Each college we can consider as a
volume and given a name and college manager write a college descriptor as belowCreation
Date
Pointer to Directory Room
Pointer to student room
Volume Name
Date When volume(college) was created
Indicates first room where student directory is stored.
Indicates first room where student is stored
User-allocated college Name
Volume Configuration - continue• Master file directory (MFD)
– Stored immediately after the volume descriptor– It lists the names and characteristics of every file
contained in that volume– File name can refer to program files, data files, and/or
system files– And if the file manager supports subdirectories, then
they’re listed in the MFD as well.• Early OS supported only a single directory per
volume. ( see the disadvantages of single directory)
Volume Configuration - continue• As an analogy
– We have many college accommodation
– Each college we can consider as a volume and given a name and college manager write a college descriptor
– And now we have master students’ directory
– Imagine if we used single directory–It will take a long time to search for students in many of the blocks and levels
–What happened if we have students with same name? Etc….
Volume Configuration - continue
• Disadvantages of single directory1. It will take long time to search for an individual file.2. IF the user had many small files stored in the
volume, the directory space would fill up before the disk storage space file up.
3. Users couldn’t create subdirectories to group the files that were related.
4. Multiple users couldn’t safeguard their files from “browsers” because the entire directory was listed on request.
5. Each program in the entire directory needed a unique name, even those directories serving many users.
Sub directories
• Most modern system provide subdirectories– File Manager create MFD that contains entries to files
and for subdirectories.– Each users given a subdirectory and they are allowed
to create subdirectories to organize their files
About Subdirectories (continued)
Figure 8.2: File directory tree structure
File descriptor• Contains information describing the file.• Typically includes
– File name – usually represented in ASCII code– File type – dependent on the system e.q files and
directories– File size– File location – where the file is stored– Date and time of creation/modification– 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.
File naming convention• Different OS has its own rule of naming convention.
Some of the rule are:-
– Character combination• E.q must start with character and then other
alphanumeric– Number of parts
• MSDOS has two parts, names and extension• UNIX has as many parts as the user need
– Length i.e number of character allows• MSDOS 6 characters• Windows 32 characters significant• Early UNIX 6 characters significant
File Naming Conventions
• Absolute filename (complete filename): Long name that includes all path info
• Relative filename: Short name seen in directory listings and selected by user when file is created
• Length of relative name and types of characters allowed is OS dependent
• Extension: Identifies type of file or its contents– e.g., BAT, COB, EXE, TXT, DOC
• Components required for a file’s complete name depend on the operating system
File Organization
• Referring to how records are arranged within a files?– Record format– Physical File Organization
• The way records are arranged and the characteristic of the medium used to store it.
File Organization: Record Format• All files composed of records that are of two types:
– Fixed-length records: Easiest to access directly• Ideal for data files• Record size critical
– Variable-length records: Difficult to access directly• Don’t leave empty storage space and don’t
truncate any characters• Used in files accessed sequentially (e.g., text
files, program files) or files using index to access records
• File descriptor stores record format
File Organization (continued)
Figure 8.4: When data is stored in fixed-length fields (a), data that extends beyond the fixed size is truncated. When data is stored in a variable length record format (b), the size expands to fit the contents, but it takes more time to access.
Physical File Organization• The way records are arranged and the characteristics of the
medium used to store them• On magnetic disks, files can be organized as: sequential,
direct, or indexed sequential• Considerations in selecting a file organization scheme:
– Volatility of the 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– Response time
• time the user is willing to wait before the requested operation is completed.
• crucial when doing searches and retrieving information in an interactive environment.
Physical File Organization (continued)• Sequential record organization: Records are stored
and retrieved serially (one after the other)– Easiest to implement – File is searched from its beginning until the requested
record is found– Optimization features may be built into system to
speed search process• Select a key field from the record and sort by the
key – Complicates maintenance algorithms
• Original order must be preserved every time records are added or deleted
Physical File Organization (continued)
• Direct record organization: Uses direct access files; can be implemented only on direct access storage devices– Allows accessing of any record in any order without
having to begin search from beginning of file– Records are identified by their relative addresses
(addresses relative to beginning of file) • These logical addresses computed when records
are stored and again when records are retrieved– Use hashing algorithms
Physical File Organization (continued)
• Advantages of direct record organization: – Fast access to records– Can be accessed sequentially by starting at first
relative address and incrementing to get to next record– Can be updated more quickly than sequential files– No need to preserve order of the records, so adding or
deleting them takes very little time• Disadvantages of direct record organization:
– Collision in case of similar keys
Physical File Organization (continued)
• Indexed sequential record organization: generates index file for record retrieval– Combines best of sequential & direct access– Divides ordered sequential file into blocks of equal
size– Each entry in index file contains highest record key
and physical location of data block – Created and maintained through ISAM software– Advantage: Doesn’t create collisions
Physical Storage Allocation
• File Manager must work with files not just as whole units but also as logical units or records
• Records within a file must have the same format but they can vary in length
• Records are subdivided into fields• Record’s structure usually managed by application
programs and not OS• File storage actually refers to record storage
Physical Storage Allocation
• Contiguous Storage– Records/information are stored one after the other in
the disk• Noncontiguous Storage
– Records/information of one files are stored in many parts of the disk
Contiguous Storage
• Records stored one after another– Advantages:
• Any record can be found once starting address and size are known
• Direct access easy as every part of file is stored in same compact area
– Disadvantages:• Files can’t be expanded easily, and fragmentation
Figure 8.7: Contiguous storage
Contiguous Allocation
Block 1
F1/1
Block 2
F1/2
Block 3
F1/3
Block 4
F2/1
Block 5
F2/2
Block 6 Block 7
F3/1
Block 8
F3/2
Block 9
F5/1
Block 10
F5/2
Block 11
F5/3
Block 12
Block 13Block 14Block 15Block 16
Noncontiguous Storage
• Allows files to use any available disk storage space• File’s records are stored in a contiguous manner if
enough empty space• Any remaining records, and all other additions to file,
are stored in other sections of disk (extents) – Linked together with pointers– Physical size of each extent is determined by OS
(usually 256 bytes)
Noncontiguous Storage (continued)
• File extents are linked in following ways:• Linking at storage level:
– Each extent points to next one in sequence– Directory entry consists of filename, storage location
of first extent, location of last extent, and total number of extents, not counting first
• Linking at directory level: – Each extent listed with its physical address, size,
and pointer to next extent– A null pointer indicates that it's the last one
Noncontiguous storage : linking at the storage level
1
F1/1 F1/3 F2/2
F1/2 F2/1
Block 13Block 14Block 15Block 16
10
10 3
3 -
12 4
4 -
File no
Start
End No.extents
1 1 3 2
2 12 4 1
directory2 -
6 - 7 -5 - 8 -
9 - 11 -
Noncontiguous storage with linking at the directory level
1
F1/1
Block 2
F1/3 F2/1
Block 5 Block 6 Block 7 Block 8
Block 9
F1/2
Block 11
F2/2
Block 13Block 14Block 15Block 16
10
3
12
4File no
address
Size Next
1/1 1 512 10
2
1/3 3 512 -
2/1 4 512 12
.
1/2 10 512 3
11
2/2 12 512 -
directory
Noncontiguous Storage (continued)
Figure 8.9: Noncontiguous file storage with linking taking place at the directory level
Noncontiguous Storage (continued)
• Advantage of noncontiguous storage:– Eliminates external storage fragmentation and need
for compactionHowever:– Does not support direct access because no easy
way to determine exact location of specific record
Indexed Storage
• Allows direct record access by bringing pointers linking every extent of that file into index block
• Every file has its own index block – Consists of addresses of each disk sector that make
up the file– Lists each entry in the same order in which sectors
are linked• Supports both sequential and direct access• Doesn’t necessarily improve use of storage space• Larger files may have several levels of indexes
Indexed Storage (continued)
Figure 8.10: Indexed storage
Access Control Verification Module
• To support file sharing among users• Need someway to specify who and what can be done to
particular files• Each file management system has its own method to
control file access• Types:
– Access control matrix– Access control lists– Capability lists– Lockword control
Access Control Matrix• Easy to implement• Works well for systems with few files & few users• Results in space wastage because of null entries
Table 8.1: Access Control Matrix
Access Control Lists
Table 8.2: Access Control List
• Modification of access control matrix technique• Each file is entered in list & contains names of users who are allowed access to it and type of access permitted
Access Control Lists (continued)
• Contains the name of only those users who may use file; those denied any access are grouped under “WORLD”
• List is shortened by putting users into categories: – SYSTEM: personnel with unlimited access to all files– OWNER: Absolute control over all files created in
own account– GROUP: All users belonging to appropriate group
have access– WORLD: All other users in system
Capability Lists
• Lists every user and the files to which each has access• Can control access to devices as well as to files
Table 8.3: Capability Lists
Which is better?
• Access Control ListFile Access
File 1 USER1 (RWED), USER2 (R-E-), USER4 (RWE-)
USER5 (--E-), WORLD (----)
File 2 USER2 (R-E-), USER3 (R-E-)
File 3
File 4
File Access
User 1 FILE1 (RWED), FILE2 (R-E-), FILE4 (RWE-)
FILE5 (--E-)
User 2 FILE2 (R-E-), FILE3 (R-E-)
User 3
User 4
Capabilities List
Lockwords
• Lockword: similar to a password but protects a single file
• Advantages:– Requires smallest amount of storage for file
protection• Disadvantages:
– Can be guessed by hackers or passed on to unauthorized users
– Generally doesn’t control type of access to file• Anyone who knows lockword can read, write,
execute, or delete file
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