Parallel disk access is offered on all BS2000 servers via Parallel Access Volumes (“static PAV”) as an alternative to single disk access (default), see the “Introduction to System Administration” [5 (Related publications)]. PAV enables the response times to be reduced in the case of high disk utilization.
Static PAV requires planning that anticipates the future device utilization, i.e. the heavily utilized devices must be assigned the right number of alias devices in advance. When hardware generation takes place in BS2000, it must be borne in mind that a separate address is required for each alias device. When an alias device is defined for each device, only 128 devices can be defined for a logical controller because a maximum of 256 devices can be connected to a logical controller.
I/O load balancing using DPAV
The IORM function DPAV (Dynamic Parallel Access Volume) is offered for disk devices connected to the Fibre Channel of /390 servers. DPAV reacts to peak loads by assigning alias devices, autonomously and dynamically, to the devices which profit most from this.
DPAV dynamically assigns alias devices which have been configured as “DPAV” devices to the heavily utilized devices. Not as many alias devices need to be generated in total any more. I/O bottlenecks caused by multiple jobs accessing the same disk are thus eased by automatically attaching alias paths.
DPAV supports Extended PAV (XPAV), see the “Introduction to System Administration” [5 (Related publications)].
Configuring DPAV devices
DPAV uses only generated alias devices.
As the device number of a base device on a type FC channel must be less than the device number of the associated alias devices (see the DVC statement in the “System Installation” manual [7 (Related publications)]), it is advisable to generate alias devices in the “rear” area of a controller.
Example
For a controller with 256 devices (device numbers 8000 through 80FF), the alias devices with the device numbers 80C0 through 80FF satisfy this condition for all base devices (device numbers 8000 through 80BF) that come into question.
However, if, for example, 8010 was generated as an alias device for 8000, it cannot be switched to a base device between 8011 and 80FF.
Alias devices on the type FC channel are switched from a base device to another base device by dynamic modification of the I/O configuration. To do this, an alias device is temporarily removed from the I/O configuration (/REMOVE-IO-UNIT) and then entered again with the same logical unit number as the new base device (/ADD-IO-UNIT). The unit address (alias address) is retained.
Activating DPAV
At first DPAV is deactivated. The DPAV function is activated using DPAV_SET_ON=YES. DPAV_DEV_ADD is used to determine alias devices for DPAV use. All alias devices which have been generated but which are not intended for DPAV can (only) be used for the static PAV.
In VM2000 mode the function must be activated in the monitor system and on each BS2000 guest system on which DPAV is to be active. The actual switchover of alias devices is coordinated and executed by DPAV in the monitor system.