- Record-oriented filesystem
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In computer science, a record-oriented filesystem is a file system where files are stored as collections of records. There are several different record formats; the details vary depending on the particular system. In general the formats can be fixed-length or variable length, with different physical organizations or padding mechanisms; metadata may be associated with the file records to define the record length, or the data may be part of the record. Different methods to access records may be provided, for example sequential, by key or by record number.
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Origin and characteristics
Record-oriented filesystems are frequently associated with mainframe and midrange operating systems, such as MVS,[1][citation needed] DOS/VSE or VMS.
Record-oriented filesystems can be supported on media other than direct access devices. A deck of punched cards can be considered a record-oriented file. A magnetic tape is an example of a media that can support records of uniform length or variable length.
In a record file system, a programmer designs the records that may be used in a file. All application programs accessing the file, whether adding, reading, or updating records share an understanding of the design of the records. In MVS there is no restriction on the bit patterns composing the data record, i.e. there is no delimiter character; this is not true of all systems, e.g., RCA File Control Processor (FCP) on the 301, 501, 601 and 3301.
The file comes into existence when a file create request is issued to the operating system. Some information about the file may be included with the create request. This information may specify that the file has fixed-length records (all records are the same size) along with the size of the records. Alternatively, specification may state that the records are of variable length, along with the maximum record length. Additional information, including blocking factor, binary vs. text, maximum number of records may be specified.
It is permitted to read only the beginning of a record; the next sequential read returns the next collection of data (record) that the writer intended to be grouped together. It may also permitted to write only the beginning of a record. In these cases, the record is padded with binary zeros or with spaces, depending on whether the file is recognized as a binary file or a text file.
Some operating systems require that library routines specific to the record format be included in the program. This means that a program originally expected to read a variable length record file cannot read a fixed length file. These operating system must provide file system utilities for converting files between one format and another. This means copying the file (which requires additional storage space, time and coordination) may be necessary.
Other operating systems include various routines and associate the appropriate routine, based on the file organization, at execution time.
In either case significant amounts of code to manage records must be provided in protected routines to insure file integrity.
An alternate to a Record-oriented file is a stream. In a stream file, the filesystem treats files as an unstructured sequence of bytes. A delimiter character ( a reserved bit pattern) may be inserted by the writer application to separate records. The read routine provides as many bytes as requested, not to exceed the size of the file. It is the responsibility of the reading application program to recognize the delimiter, not the file system library routines. This approach significantly reduces the size and complexity of the library and reduces the number of utilities required to maintain files.
Advantages and costs
A record oriented file has several advantages. After a program writes a collection of data as a record the program that reads that record has the understanding of that data as a collection. Although it is permitted to read only the beginning of a record, the next sequential read returns the next collection of data (record) that the writer intended to be grouped together. Another advantage is that the record has a length and there is no restriction on the bit patterns composing the data record, i.e. there is no delimiter character.
There is a cost associated with record oriented. The length definition takes up space. On a magnetic tape that definition takes the form of an inter-record gap. On a disk a meta data area must be allocated. This is minimal in a file where all the records are the same length. On a file composed of varying length records a maximum record length is defined to determine the size of the length metadata associated with each record.
See also
References
- ^ "z/OS V1R11 DFSMS Using Data Sets ", SC26-7410-09
Categories:- Computer file systems
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