Document revision date: 30 March 2001

## Guide to OpenVMS File Applications

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read-ahead processing: A software option used for sequentially accessing sequential files using two buffers. One buffer holds records to be read from the disk. The other buffer awaits I/O completion.

record: A set of related data that your program treats as a unit.

record access mode: The manner in which RMS retrieves or stores records in a file. Available record access modes are determined by the file organization and specified by your program.

record access mode switching: Term applied to the switching from one type of record access mode to another while processing a file.

record blocking: The technique of grouping multiple records into a single block. On magnetic tape, an IRG is placed after the block rather than after each record. This technique reduces the number of I/O transfers required to read or write the data, and, in addition (for magnetic tape), it increases the amount of usable storage area. Record blocking also applies to disk files.

record cell: A fixed-length area in a relative file that can contain a record. Fixed-length record cells permit RMS to directly calculate the record's actual position in the file.

record file address (RFA): The unique address RMS returns to your program whenever it accesses a record. Using the RFA, your program can access disk records randomly regardless of file organization. The RFA is valid only for the life of the file, and when an indexed file is reorganized, each record's RFA will typically change.

record format: The way a record physically appears on the recording surface of the storage medium. The record format defines the method for determining record length.

record length: The size of a record in bytes.

record locking: A facility that prevents access to a record by more than one record stream or process until the initiating record stream or process releases the record.

Record Management Services: See RMS (Record Management Services).

record stream: The access environment for reading, writing, deleting and updating records.

relative file organization: The arrangement of records in a file in which each record occupies a cell of equal length within a bucket. Each cell is assigned a successive number, called a relative record number, which represents the cell's position relative to the beginning of the file.

relative record number: An identification number used to specify the position of a record cell relative to the beginning of the file; used as the key during random access by key mode to relative files.

reorganization: A record-by-record copy of an indexed file to another indexed file with the same key attributes as the input file.

RFA: See record file address.

RMS (Record Management Services): The file and record access subsystem of the operating system. RMS helps your application program process records within files, thereby allowing interaction between your application program and the data.

RMS--11: A set of routines that is linked with compatibility mode and PDP--11 programs and provides similar features for RMS. The file organizations and record formats used by RMS--11 are very similar to those of RMS; one exception is that RMS--11 does not support Prolog 3 indexed files, which are supported by RMS.

root bucket: The primary routing bucket for an index; geometrically, the top of the index tree. When a key search begins, RMS goes first to the index root bucket to determine which bucket, at the next lower level, is the next link in the bucket chain.

seek time: The time required to position the read/write heads over the selected track.

sequential file organization: The arrangement of records in a file in one-after-the-other fashion. Records appear in the order in which they were written.

sequential record access mode: Record storage or retrieval that starts at a designated point in the file and continues in one-after-the-other fashion through the file. That is, records are accessed in the order in which they physically appear in the file.

shared access: A file management technique that allows more than one user to simultaneously access a file or a group of files.

stream: An access window to a file associated with a record access control block (RAB) supporting record operation requests.

stream record format: Property of a file specifying that the data in the file is interpreted as a continuous sequence of bytes, without control information, except for terminators that are recognized as record separators. Stream record format applies to sequential files only.

synchronous record operation: An operation in which your program does not regain control until after the completion of a record retrieval or storage request. See also asynchronous record operation.

terminator: Special characters or character sequences used to delimit the records in files using the stream record format.

track: A collection of blocks at a single radius on one recording surface of a disk.

tuning: The process of designing your files to achieve better processing performance.

user buffer: A buffer within an application program.

variable-length record format: Property of a file in which record length may vary.

variable-length with fixed-length control field (VFC) record format: Property of a file in which records of variable-length contain an additional fixed-length control field capable of storing data that may have no connection with the other contents of the record. VFC record format is not applicable to indexed files.

VFC record format: See variable-length with fixed-length control field (VFC) record format.

volume (disk): An ordered set of 512-byte blocks. The medium that carries Files--11 On-Disk Structure files.

volume (magnetic tape): A reel of magnetic tape, which may contain a part of a file, a complete file, or more than one file.

volume set: A collection of related volumes.

write-behind processing: A software option used for sequentially accessing sequential files using two buffers. One buffer holds records to be written to the disk. The other buffer awaits I/O completion.

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