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Chapter 9 TECHNICAL ARTICLES More About SPU Switchover/XL by Rakesh Patel Commercial Systems Division SPU Switchover/XL is one of the products within the HP high availability architecture. SPU Switchover/XL enables an SPU to assume control of information processing from another SPU for any of the following reasons: 1. The SPU must be brought down for maintenance. 2. The level of processing on the SPU has to be increased. 3. The SPU has failed. To assume processing control, the SPU requires access to switchable nonsystem (user) volume sets that ordinarily belong to the other SPU. Only one SPU can continuously access a particular switchable volume set, thereby prohibiting data corruption due to physical connections from both SPUs to the disks. Mirrored Disk/XL (30349A) can be used in conjunction with SPU Switchover/XL to provide a greater level of data availability.Figure 9-1. SPU Switchover/XL Hardware Configuration. HARDWARE CONSIDERATIONS Two HP 3000 model 9XX computers are required that should be comparable in performance and configuration capabilities. This assures sufficient computing capability so that either system can assume the duties of the other and not be overwhelmed by the additional system activities. Fiber link disks (such as the 7937FL) must form the switchable volume sets and must be connected to both SPUs. The fiber links allow each system to access the same set of disks, and also provide a closed circuit that acts as a communication path between the two systems. This path is used by each system to monitor the status of the other and to exchange systeminformation. These disks must be configured as members of nonsystem volume sets and should hold all data critical to the user's applications.
NOTE If you configure your system for DTC connected terminal switchover and network (LAN and X.25) switchover, an HP Vectra running OpenView DTC Manager is required. For information and supported configuration requirements on terminal switchover and network (LAN and X.25) switchover, contact your HP Sales Representative.
Figure 9-1 provides an illustration of hardware configuration requirements. CONFIGURATION OF SWITCH OVER ENVIRONMENT On MPE XL, system wide configuration is done using SYSGEN. SYSGEN has been enhanced to include a new first level command, SPU. This command presents a new set of second level commands made available exclusively for the SPU switch over configuration. In general, the SPU switch over configuration may be done from either of the two systems that make a pair in the switch over environment. One exception is, when systems are identified as part of the initial configuration, it is done separately on each system. The volume sets that need to be configured as switchable volume sets must be mounted on the system on which configuration will take place. This restriction suggests that one should select the proper system for performing the configuration. Another restriction is that both systems must be up and running within the same XL release operating system and the required hardware setup before they are identified as a pair with the SYSGEN configuration command, APAIR (see below). To setup the switchover environment within the current configuration, a new CI command, SPUCONTROL SETUP (see below), must be executed on the system on which the latest configuration has been created and saved. Identifying Systems If the system is being configured for switchover for the first time, it must be named to establish its identity. This is done using the ASYSTEM command at the SPU> prompt. This step must be done once on each system. The name may be the network services name that the system is known by, or any other name, provided it conform to the syntax constraints for SPU names specified by SPU Switchover/XL. For example, to identify a system as ALPHA, the command would be entered on that system as: spu> ASYSTEM system = ALPHA An analogous command must be entered on the other system to be identified as BETA: spu> ASYSTEM system = BETA Identifying System Pairs For Switchover After two systems have been identified as described, a relationship must now be established between them before the configuration can be of any use. This relationship, an ordered pair of systems, is defined using the APAIR command. The first system in the pair is called home system, and the second is called alternate system. The alternate system assumes control for the home system when a switchover is desired. However, if it is desired that each system can take over for the other, both systems must be paired such that each is both a home and an alternate system, as shown: spu> APAIR home = ALPHA alt = BETA spu> APAIR home = BETA alt = ALPHA The APAIR command automatically verifies the validity of the specified systems' names by interrogating the host system and the system connected to the host for their identities. Identifying Switchable Volume Sets The configuration is incomplete without identifying switchable volume sets. The AVOLSET command defines a switchable volume set and assigns a home for it. Only hardware configured volume sets to both systems can be considered for switchable volume sets. They must be nonsystem volume sets, but may be mirrored. The AVOLSET command must be repeated for adding additional switchable volume sets. Assigning a home system for a switchable volume set designates that the volume set would normally be mounted on this system. For a system to be accepted as a valid home, it must previously be identified by the ASYSTEM command. It must also be configured as the home system in a system pair using the APAIR command, as shown: spu> AVOLSET volset = A_DATA_SET home = ALPHA In this example, a pair must exist in the configuration with ALPHA as the home system. If not, an error message appears: **error** home system not defined in any system pair The volume set will be checked to see that it is mounted on the system where the configuration is being done. If not, an error message appears: **error** volume set not physically mounted Displaying Switchover Environment Information The switchover environment features two different coexistent states: the static state is characterized by configuration data; the dynamic state is operational and currently active. Information on the configured switchover environment can be obtained only from within the SYSGEN at SPU menu level. The SHOW command shows the currently configured switchover system pairs in the configuration file. The system pair to be shown may be specified by name in the command. If no system pair is selected, all pairs in the configuration are listed. Volume sets are listed by their home systems. The following example illustrates usage of the SHOW command: spu> SHOW Home Alternate Volume Sets ------- -------- ----------- Alpha Beta A_PRODUCTION_DATA_SET A_USER_VOL_SET A_MIRROR_DISK_SET Beta Alpha B_TEST_SOFTWARE_SET B_BENCHMARK_SET If the SHOW command is issued with the home and alt parameters, volume sets are listed as shown: spu> SHOW home=BETA alt=ALPHA Home Alternate Volume Sets ------- -------- ----------- Beta Alpha B_TEST_SOFTWARE_SET B_BENCHMARK_SET A CI command, SPUCONTROL STATUS, provides information about the state of various systems and the permanent and current residences of physically mounted switchable volume sets. The following is a sample display for SPUCONTROL STATUS when the home and alternate systems are both up: SYSTEM VOLUME SETS HOME SYSTEM ------ ----------- ----------- ALPHA A_PRODUCTION_DATA_SET ALPHA A_USER_VOL_SET ALPHA A_MIRROR_DISK_SET ALPHA BETA B_TEST_SOFTWARE_SET BETA B_BENCHMARK_SET BETA The following is a sample display for SPUCONTROL STATUS when the home system, Alpha has not been responding and the data has been switched over to the alternate system, Beta: SYSTEM VOLUME SETS HOME SYSTEM ------ ----------- ----------- BETA B_TEST_SOFTWARE_SET BETA B_BENCHMARK_SET BETA A_PRODUCTION_DATA_SET ALPHA A_USER_VOL_SET ALPHA A_MIRROR_DISK_SET ALPHA SYSTEM NOT RESPONDING: ALPHA SET UP AND ACTIVATION OF THE SWITCHOVER ENVIRONMENT With the new switchover configuration available at any time, it can be used to set up the switchover environment. This is accomplished by the CI command, SPUCONTROL, with the keyword, SETUP. With the switchover environment established between the systems, it must be activated using the same CI command, SPUCONTROL but with a different keyword, START. In doing so, the systems configured in the switchover environment would begin monitoring each other and be able to switch over the other system when desired. Once the SPUCONTROL SETUP and START keywords are executed, the switchover environment enters a state of normal operation as shown in Figure 9-2 .Figure 9-2. Normal Operation in the Switchover Environment. The SPUCONTROL SETUP The command SPUCONTROL SETUP allows us to use the latest copy of the switchover configuration for establishing the switchover environment on both systems so that each system can control and access their home volume sets. This command must be executed on the system on which the configuration has been created (or updated). As part of the setup, this system sends a copy of the switchover configuration to the other system, replacing the switchover configuration existing on it. Every time the configuration is updated, this command must be executed again in order to make the new configuration effective. The SPUCONTROL START The SPUCONTROL START command enables both systems to check the consistency of and then to activate the switchover environment. Each system begins monitoring the status of the other system by means of continuous polling. The switchover environment must already be set up, as described previously, in order to execute this command. It is important to note that after every subsequent bootup of the systems, it is necessary to re-execute this command to activate the switchover environment. UPDATING THE SWITCHOVER CONFIGURATION Three situations in which one may want to update the switchover configuration are: 1. The switchover environment is already set up and active (both SPUCONTROL SETUP and SPUCONTROL START have been executed). 2. The switchover environment is already set up but NOT active yet. 3. The switchover environment is NOT set up yet. For the first situation, it is mandatory to turn off the switchover environment before beginning to modify the current switchover configuration. This is described under "The :SPUCONTROL STOP." No special actions are required for the other situations before beginning to modify the switchover configuration. Only the deletion aspect of updating the switchover configuration information isdescribed in the following sections, since the addition part has been covered earlier under "CONFIGURATION OF SWITCHOVER ENVIRONMENT." However, all restrictions that apply during the initial configuration and described earlier under "CONFIGURATION OF SWITCHOVER ENVIRONMENT" still apply during reconfigurations (update) of the switchover environment. In order to update the switchover configuration, you must run SYSGEN and enter commands at the SPU> prompt. After reconfiguring the switchover environment, it must be set up and activated as described earlier. The SPUCONTROL STOP You can inactivate the currently active switchover environment by using the keyword STOP with the SPUCONTROL command. The immediate effect on the environment is that each system ceases to monitor the status of the other system. Other CI commands, SWITCHOVER and SWITCHBACK, become unexecutable. Deleting a Volume Set The DVOLSET command deletes a switchable volume set which was previously added to the switchover configuration with the AVOLSET command. spu> DVOLSET volset = DATA_SET If the volume set was not previously added to the configuration by the AVOLSET command, an error message appears: **error** volume set not in configuration Deleting a System Pair The DPAIR command deletes a system pair from the switchover configuration. spu> DPAIR home = BETA alt = ALPHA If the systems were set up as both home and alternate, a second DPAIR is required to delete the other system pair: spu> DPAIR home = ALPHA alt = BETA The systems must have previously been added through the APAIR command, or an error message appears: **error** system pair not in configuration Deleting a System The DSYSTEM command deletes a system from the configuration. spu> DSYSTEM system = ALPHA The system must have previously been added through the ASYSTEM command, or implicitly added by an APAIR command. Otherwise, an error message appears: **error** system not in configuration SWITCHING OVER AN SPU In an active switchover environment, as shown in Figure 9-2 , a system configured to be the alternate system in a system pair, is periodically engaged in monitoring the status of the other by means of continuous polling. For purpose of discussion, let's assume that (Alpha, Beta) is a configured system pair. If Beta detects the failure of Alpha, it would notify the operator of the problem by repeatedly displaying a message. The operator would then decide to switch over Alpha. Another possibility for switchover exists when one of the two systems, say Alpha in the above pair, operating in an active switchover environment must be brought down for maintenance. The operator may decide to switch over the switchable part of Alpha to Beta. The operator may either bring Alpha down before switching over, which in effect appears similar to the above case; or do switchover before bringing the system down, which is explained next. The final possibility for switchover exists when it has been decided to switch over the switchable part of Alpha to its alternate system, Beta, and leave Alpha up and running with its nonswitchable part. However, before executing switchover, it is necessary to close each of the switchable volume sets on Alpha. This is done by the CI command, VSCLOSE. If the operator decides to switch over Alpha, the following occurs: 1. The operator may decide to log off certain applications and sessions from the alternate system, Beta to prevent performance problems from occurring once the applications from Alpha are started on Beta. 2. The operator executes the SWITCHOVER command on Beta. This command will switch over and open each of the switchable volume sets on Beta. Once these volume sets have been mounted switchover is complete. 3. The applications that were on the home system can now be restarted on the alternate system. 4. Users can now logon to the alternate system. Figure 9-3 illusrates the switchover operation executed after SPU Alpha failed.Figure 9-3. Operation After SPU Alpha (home) Failed. For details on the SWITCHOVER command, please refer to the SPU Switchover/XL User's Guide (P/N 36378-90001). SWITCHING BACK AN SPU A switch back on a system is actually a switch over to it of its switchable part (all switchable volume sets which are configured to belong to it) from the alternate system. The alternate system holds the other system's volume sets because of a previously executed SWITCHOVER on it. The switch back is performed using another CI command, SWITCHBACK. For details on this command, please refer to the SPU Switchover/XL User's Guide (P/N 36378-90001). Assume that (Alpha, Beta) is an ordered system pair in the switchover configuration, and also assume that Alpha is switched over to Beta. Following is a typical set of activities that may be required to happen before, during, and after the SWITCHBACK is executed on Alpha: 1. The applications and sessions using the switchable volume sets of Alpha must log off Beta. 2. The operator executes on Beta, a series of VSCLOSE commands for the switchable volume sets of Alpha. 3. The operator executes the SWITCHBACK command on Alpha. 4. A message appears on both systems to indicate that switchback is occurring. 5. After the volume sets are mounted on Alpha, a message will be printed, once on Beta and repeatedly on Alpha, indicating that switchback has been completed. The operator replies to the repeating message on Alpha to acknowledge it. 6. The applications can now be restarted on Alpha. 7. Users can now logon to Alpha.
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