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File manager
Software responsible for creating, modifying, controlling access to files as well as managing the resources used by the files.
File Manager 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 as been cleared for access to it, then record its use.
4. Deallocate the file when the file is to be returned to storage, and communicate its availability to others who may be waiting for it.
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 files have no connection to other files, no dimensionality.
Database
Groups of related files that are interconnected at various levels to give users flexibility of access to the data stored. Appear as a type of file to file manager.
Program File
Contains instructions
Data file
Contains data
Directories
Special files with listings of filenames and their attributes.
Volume
A storage unit, removable or not.
Master File Directory (MFD)
Stored immediately after the volume descriptor and lists the names and characteristics of every file contained in that volume.
Subdirectory
Created when a user opens an account to access the computer system. treated as a file, but its files are filenames pointing to files.
Relative Filename
The name of a file relative to the subdirectory that it is stored in, does not include path information.
Extension
Indicates the type of a file allowing the system to properly process it.
Absolute filename
The long name that includes all path information.
Path
Information that appears in directory listings and folders
Working Directory
A subdirectory second from the home directory. Files are presumed to be located in this current directory. As long as the user refers to files in this directory they can access them without entering the complete name which the file manager does automatically. AKA current directory
Fixed-length
The most common because its the easiest to access directly. If too large, wasted space occurs. If too small, characters are truncated.
Variable Length
Records are easily read directly and do not leave wasted space or truncate. Difficult to access records directly.
Sequential record organization
records are stored and retrieved serially, one after the other. The original order must be preserved every time a record is added or deleted.
Direct record organization
Any records can be accessed in any order without searching from the beginning.
Hashing algorithm
Transforms a key field into a number: the record's logical address.
Key Field
Uniquely identifies each record.
Determines the position of a record in a stored sequence.
Collision
When two records receive the same logical address from the hashing algorithm.
Indexed Sequential record Organization
Uses hashing algorithm to generate an index file which divides record into blocks of equal size. Each entry in the index file contains the highest record key and the physical location of the block.
Contiguous Storage
Records are stored one after another. Any record can be found once its starting address and size are known. Direct access. Cannot be expanded. Fragmentation can occur over time.
Non-contiguous storage
Allows files to use any storage space available.Sequential access. eliminates fragmentation.
extents
Physical storage space where sections of a file are kept apart from the primary storage area when space is limited.
Indexed Storage
extents are listed in an index block consisting of the addresses of each disk sector that make up the file. Extents are listed in the same order that they are linked
Current Byte Address (CBA)
A byte indicating the address of the last byte read and used to access the next sequential record. Must be updated every time a record is accessed.
Access Control Matrix
Each column identifies a user and each row identifies a file. The intersection contains the access rights for the user to the file. Can grow extremely large. Wastes space with null entries.
Control Access List
each file is entered in the list and contains the names of the users who are allowed to access it and the type of access each is permitted.
Capability List
Lists every user and the files to which each has access. In Linux/UNIX they control access to device as well as to files.
Data Compression
Algorithms consist of two type: lossless which retain all the data in the file throughout the compression/decompression process.; Lossy which remove data permanently. Space is gained but processing time is lost.
File descriptor
information kept in the directory to describe a file or file extent.
Relative address
In a direct organization environment, it indicates the position of a record relative to the beginning of the file.
