HP OpenVMS Systems Documentation

Guide to OpenVMS File Applications

As described in Chapter 4 and Chapter 6, the program should specify the specification for the file being opened (or created) and can also specify default file specifications. The file specifications are summarized in the following table:

A number of run-time options that open files and connect record streams can collectively improve application performance. Such options include the buffering options discussed in Chapter 7.

Two run-time performance options not discussed previously are particularly important when adding records to a file: extension size and window size.

9.2.3.1 Extension Size

If you intend to add records to the file, specify a reasonable default extension size to reduce the number of times the file is extended.

Use the Edit/FDL utility to calculate the correct extension size. The Edit/FDL utility uses your responses to assign an optimum value for the FDL attribute FILE EXTENSION. With multiple area files, the Edit/FDL utility assigns optimum values to the AREA EXTENSION attributes.

If you do not specify an extension size, RMS computes the size; however, this size may not be optimum.

If you decide to create an FDL file for defining an indexed file without using EDIT/FDL, you can approximate the value of the EXTENSION attributes. You do this by multiplying the number of records per bucket by the number of records that you intend to add to the file during a given period of time.

To see the current default extension size, use the DCL command SHOW RMS_DEFAULT. To set the default buffer count, use the DCL command SET RMS_DEFAULT/EXTEND_QUANTITY=n, where n is the number of blocks per extension. The corresponding field is FAB$W_DEQ.

9.2.3.2 Window Size

If the file is extended repeatedly, the extensions may be scattered on the disk. Each extension is called an extent---a pointer to each extent resides in the file header. For retrieval purposes, the pointers are gathered together in a structure called a window. The default window size is 7 pointers, but you can establish the window size to contain as many as 127 pointers. You can also set the window size to --1, which makes a window that is just large enough to map the entire file.

When you access an extent whose pointer is not in the current window, the system has to read the file header and fetch the appropriate window. This is called a window turn, and it requires an I/O operation.

Window size is a run-time option. Many high-level languages include a clause that sets window size when a file is opened.

You can set the window size (FAB$B_RTV field) at run time with a VAX MACRO subroutine or with the FDL attribute FILE WINDOW_SIZE.

You can increase the default window size for a specific volume by using the DCL commands MOUNT and INITIALIZE. However, using additional window pointers increases system overhead. The window size is charged to your buffered I/O byte count quota, and indiscriminate use of large windows may result in exceeding the buffered I/O byte count quota or may exhaust the system's nonpaged dynamic memory.

You can use the Backup utility (BACKUP) to avoid having too many extents. When you restore a file, BACKUP tries to write the file in one section of the disk. Although BACKUP does not necessarily create a contiguous copy of the file, it does reduce the number of extents. If you are regularly backing up the file, the number of extents is probably reasonable. For more information about BACKUP, see the OpenVMS System Manager's Manual.

Where disk space is available, you can reduce the number of extents by creating a new, contiguous version of the file using either the Convert utility (CONVERT) or the DCL command COPY/CONTIGUOUS. If neither of these conditions apply, a larger window size is the only option to use. For file maintenance information, see Chapter 10.

9.2.3.3 Summary of Performance Options

The following table summarizes the run-time open and connect options that may affect performance:

You can specify the record access for a record stream as sequential, random by key or record number, or random by RFA. (See Section 8.1.) The selected record access can be changed for each record processing operation. These options can be set using the RAB$B_RAC field, values RAB$C_SEQ, RAB$C_KEY, and RAB$C_RFA.

9.2.5 Options for Adding Records

When adding records to a file, consider the open and connection options in the following table:

The following table lists the run-time file open options that apply to data reliability:

The run-time file open options that apply to file disposition are listed in the following table. These options can only be selected while the file is open.

The following table lists the run-time options that apply to indexed file processing. For more information about processing indexed files, refer to Section 8.4.3.

The run-time file open and close options that apply to magnetic tape processing are listed in the following table:

The following table lists the run-time file open options that apply to nonstandard file processing: