Creating paging files
Paging files are cataloged under the system administration user ID with the name SYS.PAGING.<vsn>. A paging file may occupy more than one extent on the volume with the specified volume serial number vsn. In a pubset it is possible for precisely one SYS.PAGING.<vsn> paging file to exist on each volume.
A paging file has a minimum size of 1 MB. For reasons of performance it is recommended that the paging area be distributed over several paging files (i.e. over multiple volumes) and that all these paging files be more or less the same size.
Paging files can be created during a session with the CREATE-PAGING-FILE command. This command sets up a paging file SYS.PAGING.<vsn> of the specified size. It is possible to select each disk of an imported pubset via the VSN.
Assigning paging files to the paging area
An existing paging file can be brought into service, i.e. assigned to the paging area, using the EXTEND-PAGING-AREA command. For it to be used right from the start in following sessions it must be entered in the system parameter file (see the PAGING parameter record in section "Selection of the paging area at startup (PAGING)").
Paging files can also be created using the SIR statement CREATE-PAGING-FILE. SIR is described in the “Utility Routines” manual [15].
It makes sense always to create the paging files on the home pubset on system installation since operating system requirements are automatically satisfied in this way.
The required size of the paging area depends on the number and virtual program size of the programs which run simultaneously. To this must be added the system address space currently set.
The following formula is recommended for an initial evaluation of space requirements:
required space = 2 * (number of user tasks * virtual program size + system address space)
Once operation has begun, this value should be checked (e.g. using the SHOW-PAGING-CONFIGURATION command or openSM2; see the “openSM2” manual [49]) to ascertain the actually required size of the paging area (see also the “Performance Handbook” [37]).
Releasing paging files and partitions from the paging area
The REDUCE-PAGING-AREA command can be used to release a paging file or partition from the paging area during operation.
However, this is only possible if the existing paging area is not yet exhausted and the system will not find itself in a saturation state as a result of the reduction. The REDUCE-PAGING-AREA command executes asynchronously in a server task.
Measures for accelerating the release of paging files are described on "Measures for accelerating the release of paging files".
Deleting paging files
The DELETE-PAGING-FILE command is available for deleting a paging file. For this to be possible, the associated pubset must have been imported.
It takes a relatively long time to execute the DELETE-PAGING-FILE command because the file contents are deleted (overwritten with binary zeros). The command therefore runs asynchronously in a server task.
When deleting a paging file in the home pubset it is necessary to observe the following: Deletion with DELETE-PAGING-FILE is rejected if not enough paging files are left in the home pubset.
Dynamic reconfiguration of the paging area
The paging area can be extended dynamically at any time during the session using the EXTEND-PAGING-AREA command. To permit this, the pubset with the paging file to be included must be imported and the maximum size of the paging area (4 TB) must be observed.
The REDUCE-PAGING-AREA command enables the paging area to be reduced dynamically. A few requirements must be observed here, such as whether the remaining size of the paging area is sufficient for the tasks that are to be performed.
You are recommended to make extremely careful use of paging area reconfiguration during ongoing operation and to restrict it to a minimum. Frequent reconfiguration can reduce the overall system performance more and more. This applies particularly for systems with a high paging load.
The following mode of operation enables you to maintain a good overall performance:
Restrict the procedure to paging files which will presumably only rarely or never be removed from the paging area.
The use of contiguous paging files is recommended.
It is better to use a large number of small disks than a few large ones.
It is recommendable to use these disks exclusively for paging files (pure paging pubsets).
Measures for accelerating the release of paging files
If the paging file to be released is almost fully utilized, relocating the virtual pages to other paging files will result in longer execution times for commands, a higher I/O workload and a higher load on the CPU. To reduce these loads or to simplify the reduction process, the following options are available:
The reduction is reported to the operating system some time in advance.
The MODIFY-PAGING-AREA-ATTRIBUTES command (UTILIZATION=*LOW operand) is issued and, from this point on, the affected paging file is – as far as possible – no longer used for adding virtual pages. Through access to pages within the paging file or through page release, the paging file becomes emptier, and for the actual reduction fewer I/Os are required for transferring these pages.
This method does not guarantee success: either the system cannot forego reading in pages to this paging file (because the paging rate is too high) or the pages within the paging file are not accessed.
Whether or not this method has been successful can be established using the SHOW-PAGING-CONFIGURATION command: an increase in the displayed free size of the paging file is a measure of success and shows when the reduction is to be started. This can be in a few minutes or a few hours.
Already when the EXTEND-PAGING-AREA command (operand UTILIZATION=*LOW) is used, the operating system is informed that this paging file should be used less for adding virtual pages. The likelihood of success is the same here as in the previous point.
By specifying the operand LATER-REDUCTION=*REQUESTED in the EXTEND-PAGING-AREA command you can lessen the CPU load during the reduction. However, more resident working memory is required to manage these paging areas.
You are advised to limit all the above measures to no more than two paging files.