Virtual Device Engine › Virtual Device Channel Paths

Virtual Device Channel Paths

When the Virtual Devices are defined in the I/O Definition File (IODF), channel paths (CHPIDs) are not assigned to the devices. Instead CHPIDs are automatically assigned to the devices by CA Vtape during the first start up after an IPL. CHPID assignment is critical since the operating system architecture will not allow an I/O device to be varied online if it does not have a CHPID assigned.

The CHPIDs chosen for automatic assignment to the Virtual Devices are those associated with the DASD volume on which the BSDS1 is allocated. All the CHPIDs associated with the BSDS1 DASD volume will be assigned to each Virtual Device. A maximum of eight CHPIDs per Virtual Device are supported.

Automatic CHPID assignment can be manually overridden by coding either a CHPID DD in the SVTSCE PROC or specifying the ChpidDeviceList attribute in the Virtual Control Unit section of parmlib. The preferred method is the ChpidDeviceList attribute.

The CHPID DD can be coded in the following ways:

• //CHPID DD DISP=SHR,UNIT=SYSALLDA,VOL=SER=vvvvvv, where vvvvvv is a DASD volume whose CHPIDs are to be used.
• //CHPID DD DISP=SHR,DSN=your.data.set, where your.data.set has been allocated on the DASD volume whose CHPIDs are to be used.
• //CHPID DD DISP=SHR,UNIT=(/uuuu,,DEFER),VOL=SER=VTAPES, where /uuuu is the four-digit unit address (preceded by a forward slash) of the tape drive whose CHPIDs are to be used.

Note: When trying to use a tape CHPID, the unit specified on the CHPID DD must be online to the system on which CA Vtape is being started.

The ChpidDeviceList attribute can be coded with a list of online devices who's CHPIDs should be used as the Virtual Device CHPIDs.

After the Virtual Device CHPIDs are assigned, automatically or manually, they are remembered across CA Vtape warm starts. Adding or removing the CHPID DD or the ChpidDeviceList attribute has no effect until the next cold start. Cold starts only occur after the system is IPLed.

If the Virtual Device CHPIDs must be changed, stop all CA Vtape virtual tape activity, stop CA Vtape, change the VirtualDeviceList attribute in the Parmlib Directory Section of the OSPARMS member to point to the VTDRALT member, and start CA Vtape. The VTDRALT member contains an unused set of device addresses defined for use by CA Vtape during initial customization. Since these devices have never had CHPIDs assigned to them, CA Vtape can assign CHPIDs to these devices and vary them online without violating the operating system architecture and without the need for an IPL.

Overriding the default CHPID choice may be required due to a channel problem, a DASD problem, or a microcode upgrade to DASD hardware that causes the hardware to take its CHPIDs offline during the upgrade process. When overriding the default CHPID choice, the best CHPID to pick is the one that has the least likelihood of being taken offline. Since the CHPID is only assigned to satisfy the operating system architecture and is not used to perform any I/O's, the CHPID chosen will not experience an increase in I/O activity that could degrade the performance of the other devices that actually use the CHPID for I/O.

If you modify the CHPID DD to point to a different device and you restart CA Vtape without an IPL, and continue using the same Virtual Devices, the devices may fail to vary online. The system always adds new CHPIDs to the already-defined CHPIDs and as a consequence the maximum number of eight CHPIDs may be easily exceeded. In this situation the only solution is to use the alternate set of Virtual Devices (member VTDRALT) or IPL the system.

An alternative solution would be to set up and run two different CA Vtape Subsystems sharing the same Global VCAT and BSDS1. The SVTSCE PROC would be copied to create an SVTSCE1 and an SVTSCE2 PROC. Each PROC would have a CHPID DD coded. The CHPID DD's would reference DASD volumes in different hardware subsystems so that all the Virtual Devices are not using the same CHPIDs.

This would allow you to vary offline the Virtual Devices for SVTS1 which are using the CHPID's associated with one DASD hardware subsystem. A microcode upgrade which takes the CHPID's offline could then be safely performed on that DASD hardware subsystem. Once the upgrade was complete, the SVTSCE1 Virtual Devices could be varied online. The SVTSCE2 Virtual Devices would then be varied offline and the other DASD hardware subsystem upgrade would be performed. When that upgrade is complete, the SVTSCE2 Virtual Devices could be varied online.

We recommend specifying the ChpidDeviceList attribute to assign non-DASD CHPIDS to Virtual Devices. This avoids having to set up multiple subsystems and prevents CA Vtape Virtual Devices from being involved in any DASD channel recovery events.

Note: DASD subsystems containing paging or XCF data sets should be avoided. The best candidates are CTC (Channel-To-Channel) and OSA (Open Systems Adapter) CHPIDs.

A best practices approach is to spread the Virtual Devices across multiple Virtual Control Unit address spaces and then spread the CHPID utilization across multiple non-DASD devices using the ChpidDeviceList attribute.