DCL:MISC£±:Summary:File Structures
Furthermore, when reading or writing, it is generally more efficient for the
computer to perform the operation collectively in units that are as large as
possible. Thus, the processing speed for blocked structures are usually
faster.
Normally, the VBS structure is used for the unformatted SEQUENTIAL file. The VBS structure seems to be most consistent with the FORTRAN standard, and so in many cases, this structure is selected when no specifications have been made, such as with the DD statement.Since formatted files are often outputted to devices which have a specific length, F and FB structures are also used. In this case, the user must be careful not to let the maximum record length written by the program exceed the file record length.
Outputting unformatted data to F and FB structured files is forbidden since it is extremely inconsistent with the FORTRAN grammar. However, they may be treated as files that contain control codes. In any case, for a user to specify the block length, he will be required to have knowledge on the system administrator level, far exceeding that of a FORTRAN programmer, and is troublesome.
However, from the opposite perspective, this means that it is possible to tune-up programs to the limit by finding the right combination for the nature of the processing performed by the program and the system characteristics. In mainframe- system computers, there are actually significant differences in the processing speed depending on the selection of parameters such a block length.
Only fixed-length record files can be used as DIRECT files. Since DIRECT files can be randomly accessed, they cannot be blocked. Thus, the SEQUENTIAL file is much faster in sequential access of records.Generally, DIRECT files are not easy to handle in mainframe-system computers, and so are not frequently used.