Configuring Multi-Target Geographic Mirroring (7.5)

Background

Geographic Mirroring supports replication between up to six nodes across two sites. Replication within a site is synchronous; replication across sites can be either synchronous or asynchronous.

Before you begin

This scenario assumes that the following tasks have been completed prior to these steps:

A cluster between the nodes has been created, and the cluster nodes have a status of Active.
The nodes are all in the same device domain. When creating a cluster or adding a node to a cluster with default options, PowerHA automatically adds nodes to a single device domain.
An IASP has been created on the production node.

As part of this process, when configuring Geographic Mirroring between the first target system, the production copy of the IASP will need to be varied off. Configuring and deconfiguring Geographic Mirroring between additional nodes does not require the production IASP to be varied off.

Procedure

Create the Cluster Resource Group

A cluster resource group (CRG) defines the following:

A recovery domain is a list of nodes that are potential hosts for the IASP along with:
- The current role the node has (primary or backup)
- The data port IP address(es) to use for replication.
A list of configuration objects, such as IASPs or IP addresses, that are switched between the nodes.

A CRG can be created using any of the following methods:

Type the CRTCRG command and press F4 to go to the Create Cluster Resource Group (CRTCRG) command screen.
Using the Work with Cluster (WRKCLU) command menu. Select option 9, Work with cluster resource groups, and use option 1, Create, and enter a name for the CRG to go to the Create Cluster Resource Group (CRTCRG) command screen.
From the command line, enter the CRTCRG command and your parameters.

For this example, a CRG is created using the following command:

CRTCRG CRG(GEOCRG) 
       CRGTYPE(*DEV) 
       RCYDMN((MN01 *PRIMARY *LAST MN ('192.0.2.100' '192.0.2.101'))
              (MN02 *BACKUP  1     MN ('192.0.2.110' '192.0.2.111'))
              (IA01 *BACKUP  2     WI ('192.0.2.200' '192.0.2.201'))
              (IA02 *BACKUP  3     WI ('192.0.2.202' '192.0.2.203'))) 
       CFGOBJ((MYIASP *DEVD *ONLINE))

This command creates a device CRG called GEOCRG. The CRG defines four nodes: two in a site named MN and two in a site named WI. Each node has two data port IP addresses defined for replication. The CRG has a single IASP device specified that is switched with the CRG called MYIASP.

Data Port IP Addresses

It is possible to use anywhere from 1 to 4 dataport IP addresses per node. Dataport IP addresses are used for Geographic Mirroring data replication. While only a single data port IP address is required, it is generally recommended to have more than one per node for improved performance and throughput. The greatest performance benefit is typically noticed in going from one data port IP address to two.

Automatic Failover

By default, a cluster resource group has automatic failover enabled. When a failure is detected, automatic failover will automatically switch the IASP to a backup node.

Disabling Automatic Failover

Automatic failover can be disabled using the QCST_CRG_CANCEL_FAILOVER PowerHA policy. The following example disables all automatic failover for the CRG named GEOCRG:

ADDHAPCY PCY(QCST_CRG_CANCEL_FAILOVER) PCYDMN(GEOCRG) QUAL('SCOPE(*CRG)') VALUE('EVENT(*ALL)')

For additional information on customizing automatic failover, see the QCST_CRG_CANCEL_FAILOVER policy.

Enhanced Automatic Failover with Cluster Monitor

In environments where automatic failover is desired, it is possible to enhance automatic failover to monitor for additional events via a Hardware Management Console (HMC) using cluster monitors. See Advanced Node Failure Detection for additional information.

Optional CRG Exit Program

CRG Exit programs provide a way to define a program to call whenever a cluster event occurs and enable enhanced automation. See the Cluster Resource Group Exit Program topic in the IBM i Documentation for more information

Configure Geographic Mirroring

Configuring the First Target Node

As part of this process, when configuring Geographic Mirroring between the first target system, the production copy of the IASP will need to be varied off. Configuring and deconfiguring Geographic Mirroring between additional nodes does not require the production IASP to be varied off.

Vary off the IASP on the production system if it currently has a status of Available by using the VRYCFG command. For example: VRYCFG CFGOBJ(MYIASP) CFGTYPE(*DEV) STATUS(*OFF)
Use the Configure Geographic Mirroring (CFGGEOMIR) command to configure Geographic Mirroring replication to the first node. For this example, the following command is used:
```
CFGGEOMIR ASPDEV(MYIASP) 
          ACTION(*CREATE)
          DELIVERY(*SYNC) 
          COMPRESS(*RESYNC) 
          TGTNODE(MN02)                                                        
```
This command configures Geographic Mirroring for the IASP named MYIASP to the node MN02.
Since MN02 is within the same site as the production system, delivery must be *SYNC.
A value of *RESYNC is specified for transmission compression. Transmission compression can reduce the time required for initial synchronization and resynchronization and uses hardware-accelerated compression for Power10 and later systems. On systems prior to Power10, software compression is used. Compression can have higher CPU overhead on the production system.

Since *SELECT was specified for the disk units, a panel showing all non-configured units is displayed on the target node, as shown below. Use option 1 to select the disk units you want from the list.

PowerHA                Select Non-Configured Disk Units                       
                                                                              
ASP device . . . . . . :   MYIASP        Mirror copy node . . . :   MN02
Prod copy capacity . . :   28634         Mirror copy capacity . :   0         
Prod disk units  . . . :   3             Mirror disk units  . . :   0         
                                                                              
Type options, press Enter.                                                    
  1=Select                                                                    
                                                                              
     Resource                                                                 
Opt  Name        Serial Number    Type Model  Capacity  Rank  Eligible               
 1   DMP476      YF5258000079     2145 0050       9544  002      Yes          
 1   DMP481      YF525800007A     2145 0050       9544  002      Yes          
 1   DMP485      YF525800007B     2145 0050       9544  002      Yes          
                                                                       More...
F1=Help   F9=Calculate Selection   F11=View 2   F12=Cancel

After confirming the selection, PowerHA will initialize and format the disk units on the target system, configure Geographic Mirroring, and create copy descriptions and a session to manage the replication. The default session name if *GEN is used is GEOMIRSSN.
Since the default value for the start indicator parameter was left as *YES in the command, PowerHA will also start the cluster resource group, vary on the IASP on the production system, and start replication.

Configuring Additional Target Nodes

Since Geographic Mirroring is already configured between two nodes, there is no need to vary off the IASP on the production system. This enables users and applications to be using the IASP on the production system during the rest of this process.

End the cluster resource group to configure replication to the next node. For example: ENDCRG CRG(GEOCRG)
Use the Configure Geographic Mirroring (CFGGEOMIR) command to configure Geographic Mirroring replication to the additional node. For this example, the following command is used:
```
CFGGEOMIR ASPDEV(MYIASP) 
          ACTION(*CREATE)
          DELIVERY(*ASYNC) 
          COMPRESS(*RESYNC) 
          TGTNODE(IA01)                                                        
```
This command configures Geographic Mirroring for the IASP named MYIASP to the node IA01.
Since IA02 is in a different site from the production system and is the lowest numbered node, it is possible to specify asynchronous delivery, enabling further distances between the nodes.
Since *SELECT was specified for the disk units, a panel showing all non-configured units is displayed on the target node. Use option 1 to select the disk units you want from the list.
After confirming the selection, PowerHA will initialize and format the disk units on the target system and configure Geographic Mirroring.
Since the default value for the start indicator parameter was left as *YES in the command, PowerHA will also start the cluster resource group, vary on the IASP on the production system if it was not already varied on, and start replication.
This example would repeat this process starting at step 1 for the final node IA02. With IA02, a delivery value of *SYNC must be specified since synchronous replication will be used between the systems within the IA site containing nodes IA01 and IA02.

Results

Geographic Mirroring is now configured between the nodes in the CRG. To verify and monitor the status of replication, use the Display ASP Session (DSPASPSSN) command. For example, DSPASPSSN SSN(GEOMIRSSN).