The task of I/O configuration in general and z/OS Hardware Configuration Definition (HCD) especially is to allow customers to run their IBM Z servers 24 by 7. HCD is used to define the configuration of the IBM Z server (hardware configuration) and the configuration of the I/O for the operating systems running on these servers (software or operating system configuration). Whenever any change in configuration is necessary HCD performs a so-called dynamic activation. This changes the configuration while the server and the operating systems on it remain running. To support best availability HCD calculated the changes that need to be applied to get to a new target configuration in the least disruptive way. Since you’re starting the process from within HCD running in z/OS, the process involved hardware and software configuration changes.
This article describes latest improvements in the area of hardware-only dynamic activation and the dependencies between z/OS and firmware.
What is HCD hardware-only dynamic activation
HCD is the z/OS and z/VM program to dynamically activate hardware and software configuration changes for ages. To apply hardware changes, HCD needs to be invoked on at least one z/OS or z/VM operating system instance on the server to modify. There are IBM Z servers that are usually not running any instance of a z/OS or z/VM operating system, for example a Linux server or a stand-alone coupling facility (SACF). For Linux, there is the possibility to use IBM Z Dynamic Partition Manager (DPM) instead of HCD. Nevertheless, many customers do data center hardware configuration in HCD. Such servers could not be dynamically activated by HCD. Therefore, any I/O configuration change required a Power-On Reset (POR). This is disruptive and especially in case of a SACF includes many additional actions to remain operational during the POR.
To solve this issue IBM Z firmware and HCD for z/OS developed a way to dynamically activate hardware configuration changes for servers not running an instance of z/OS or z/VM. Since there is no software configuration involved in that process it’s called hardware-only dynamic activation in HCD.
Initial functionality z14 GA 2
The support for hardware-only dynamic activation in the firmware was first delivered with z14 GA2. That is therefore the minimum hardware requirement. The z/OS support was delivered also for the z14 GA2 with APAR OA54912 for HCD.
To run the hardware-only activation on the IBM Z server, an activation service on the server is required. Because the functionality was delivered in mid release of the hardware server the activation service was introduced deactivated. Therefore, an enablement operation had to be done. The activation service is running in one of the IBM reserved partition on the z14 server. This partition must be defined as Partition ‘B’ in the highest channel subsystem of the server, and it must be named MCS_1. Because without that service no dynamic change is possible, activating this hardware configuration change requires the last POR for the server.
After the partitions has been defined and the server being PORed it is now able to remotely be triggered to perform hardware I/O configuration changes. Whenever a configuration change is necessary you build a new I/O Definition File (IODF) containing the target configuration. To apply the new configuration to the remote server, you start in the HCD processor list panel, HCD Option 2.11. There, you have options to check the currently active configuration and to activate the current IODF. Displaying the current configuration is like displaying the currently active configuration for the current processor. The different is that for the local processor, you also get the currently active software configuration of your operating system instance and information about the hardware configuration. The remote server does not run any operating system with a software configuration and therefore does not contain that information.
When activating a new IODF at the remote server you can perform an activate that looks very similar to the way an activate I/O configuration change looks on your local server. You have the same options as test activate, real activate, force options, etc. as you already know from HCD for other dynamic activate options. The difference here is again, that you do not specify options for software changes because there is no software running on the server that support that.
Once you start the activation process, HCD takes the IODF of the active configuration and the target configuration of the remote server, sends them over to the remote server via a connection through the processor LAN, and the activation service performs the dynamic changes in the same way HCD would do.
That way, you could dynamically activate any hardware configuration change on the remote server. Because of limited test capacity in IBM at that time, the functionality has been fully tested only for coupling I/O. Therefore, the hardware-only dynamic activation support is currently limited to SACFs. That’s why it is sometime also called dynamic I/O for stand-alone coupling facilities.
Updates over time
Some customers do their I/O configuration on multiple systems having different high-level qualifiers for the IODF name when building a production IODF. The IODF name gets added as descriptors to the configuration when activating a configuration. If you are then trying to activate from a system with different IODF naming convention, then HCD cannot find the active configuration under the expected name on the local system. With APAR OA58145 HCD has been enhanced to allow you to override the name of the IODF containing the active configuration for the target server. Since this is not needed if HCD can find the IODF on the local z/OS system, HCD allows overriding the name only in case the active configuration cannot be found.
With z14, the firmware functionality has been delivered mid of the lifetime and therefore has been introduced disabled. With the follow-on server, the IBM z15, the functionality has been enabled from the beginning. HCD always generates the definition of the MCS_1 partition. There is no way anymore that you miss this functionality, and you can dynamically activate configuration already from the start.
With APAR OA59075 HCD delivered an update for the hardware-only dynamic activate because of an incompatible change in the hardware activation service on z15. This service was updated for internal reasons. With the update, the overall process has been enhanced in a way that also includes some benefits for customers.
Since the activation service is incompatible, the z/OS HCD change has been released more than half a year before the firmware change was delivered. This gives you enough time to install the z/OS PTFs for the updated firmware. The firmware for the updated activation service requires MCL P46613.002 to be installed on your z15 Support Element Driver D41C with Bundle S51a.
The main benefit that can be experienced once the service is installed is that the process to activate hardware I/O configuration changes should now be much faster. Because there is no standard configuration that can be used to compare dynamic configuration changes, there are no official performance numbers. Nevertheless, we experienced much more stable activation times across different configuration sizes and the elapsed time for the activation drops significantly. Internal tests show improvements by factors. Depending on the size of your IODF, your benefits can vary but may be even greater.