Using Pacemaker-based Red Hat Enterprise Linux HA with Caché

Red Hat Enterprise Linux

When configuring Linux on the nodes in the cluster, use the following guidelines:

• All nodes in the cluster must have identical userids/groupids (that is, the name and ID number must be identical on all nodes); this is required for Caché.

  The following users must be added and synchronized between nodes:

  • Owner(s) of the Caché instance(s)

  • Effective user(s) assigned to each instance’s Caché jobs

  The following groups must be added and synchronized between nodes:

  • Effective group(s) to which each instance’s Caché processes belong

  • Group(s) allowed to start and stop the instances

• All volume groups required for Caché and the application are available to all nodes.

• All fully qualified public and private domain names must be included in the hosts file on each node.

Red Hat Enterprise Linux High Availability Add-On

This document assumes RHEL 7 or later, with the HA Add-On using Pacemaker as the high availability service management component. The script and directions here apply only to Pacemaker-based HA. RHEL 6.5 includes Pacemaker-based HA and may work as well; consult with Red Hat and the InterSystems Worldwide Response Center (WRC)Opens in a new tab for guidance.

In general, you will follow these steps:

1. Install and cable all hardware, disk and network.

2. Configure STONITH fencing resources.

3. Create VIP and disk resources (file system, LVM, perhaps CLVM) that include the network paths and volume groups of the shared disk.

Be sure to include the entire set of volume groups, logical volumes and mount points required for Caché and the application to run. These include those mount points required for the main Caché installation location, your data files, journal files, and any other disk required for the application in use.

With the move to Pacemaker, the RHEL HA Add-on no longer supports qdisk or any quorum disk. With two-node clusters, therefore, a robust STONITH configuration is especially important to avoid a partitioned or unsynchronized cluster. Consult with Red Hat on other possibilities such as adding a third-node for quorum purposes only.

Installing a Single Instance of Caché

To install a single instance of Caché in the cluster, use the following procedure.

1. Bring the resource group online on one node. This should mount all required disks and allow for the proper installation of Caché.

   1. Check the file and directory ownerships and permissions on all mount points and subdirectories.

   2. Prepare to install Caché by reviewing the “Installing Caché on UNIX® and Linux” chapter of the Caché Installation Guide.

2. Create a link from /usr/local/etc/cachesys to the shared disk. This forces the Caché registry and all supporting files to be stored on the shared disk resource you have configured as part of the resource group.

   A good choice is to use a ./usr/local/etc/cachesys/ subdirectory under your installation directory. For example, assuming Caché is to be installed in /cacheprod/cachesys/, specify the following:

   mkdir –p /cacheprod/cachesys/usr/local/etc/cachesys
   mkdir –p /usr/local/etc/
   ln –s /cacheprod/cachesys/usr/local/etc/cachesys /usr/local/etc/cachesys
   

   

3. Run Caché cinstall on the node with the mounted disks. Be sure the users and groups (either default or custom) are available on all nodes in the cluster, and that they all have the same UIDs and GIDs.

4. Stop Caché and relocate all resources to the other node. Note that Pacemaker does not yet control Caché.

5. On the second node in the cluster, create the link in /usr/local/etc/ and the links in /usr/bin for ccontrol and csession:

   mkdir –p /usr/local/etc/
   ln –s /cacheprod/cachesys/usr/local/etc/cachesys/ /usr/local/etc/cachesys/
   ln –s /usr/local/etc/cachesys/ccontrol /usr/bin/ccontrol
   ln –s /usr/local/etc/cachesys/csession /usr/bin/cession
   

   

6. Manually start Caché using ccontrol start. Test connectivity to the cluster through the VIP. Be sure the application, all interfaces, any ECP clients, and so on connect to Caché using the VIP as configured here.

7. Be certain Caché is stopped on all nodes.

8. Add the Caché resource configured to control your new instance to your cluster, as follows. This example assumes the instance being controlled is named CACHEPROD. See Understanding the Parameters of the Caché Resource for information about the Instance and cleanstop options.

   pcs resource create CacheProd ocf:heartbeat:Cache Instance=CACHEPROD cleanstop=1
   

   

9. The Caché resource (CacheProd) must be configured to collocate and ordered to start after its disk resources and optionally its VIP resource(s). To prevent unexpected stops and restarts, Caché and its collocated resources should be configured to prefer their current location (resource-stickiness=INFINITY) and thus never fail back after a node reboots.

10. Verify that Caché starts in the cluster.

Installing Multiple Instances of Caché

To install multiple instances of Caché, use the procedure in this section.

To install Caché on the first node, do the following:

1. Bring the resource group online on one node. This should mount all required disks and allow for the proper installation of Caché.

   1. Check the file and directory ownerships and permissions on all mount points and subdirectories.

   2. Prepare to install Caché by reviewing the “Installing Caché on UNIX® and Linux” chapter of the Caché Installation Guide.

2. Run Caché cinstall on the node with the mounted disks. Be sure the users and groups (either default or custom) are available on all nodes in the cluster, and that they all have the same UIDs and GIDs.

3. The /usr/local/etc/cachesys directory and all its files must be available to all nodes at all times. To enable this, after Caché is installed on the first node, copy /usr/local/etc/cachesys to each node in the cluster. The following method preserves symbolic links during the copy process:

   cd /usr/local
   rsync -av -e ssh etc root@node2:/usr/local/
   

   

4. Verify that ownerships and permissions on the cachesys directory and its files are identical on all nodes

   Note:

   In the future, keep the Caché registries on all nodes in sync using ccontrol create or ccontrol update, or by copying the directory again; for example:

   ccontrol create CSHAD directory=/myshadow/ versionid=2015.1.475
   

   

5. Stop Caché and relocate all resources to the other node. Note that Pacemaker does not yet control Caché.

6. On the second node in the cluster, create the links in /usr/bin for ccontrol and csession, as follows

   ln –s /usr/local/etc/cachesys/ccontrol /usr/bin/ccontrol
   ln –s /usr/local/etc/cachesys/csession /usr/bin/csession
   
   

   

7. Manually start Caché using ccontrol start. Test connectivity to the cluster through the VIP. Be sure the application, all interfaces, any ECP clients, and so on connect to Caché using the VIP as configured here.

8. Be certain Caché is stopped on all nodes.

9. Add the Caché resource configured to control your new instance to your cluster, as follows. This example assumes the instance being controlled is named CACHEPROD. See Understanding the Parameters of the Caché Resource for information about theInstance and cleanstop options.

   pcs resource create CacheProd ocf:heartbeat:Cache Instance=CACHEPROD cleanstop=1
   

   

10. The Caché resource (CacheProd) must be configured to collocate and ordered to start after its disk resources and optionally its VIP resource(s). To prevent unexpected stops and restarts, Caché and its collocated resources should be configured to prefer their current location (resource-stickiness=INFINITY) and thus never fail back after a node reboots.

11. Verify that Caché starts in the cluster.

When you are ready to install a second instance of Caché within the same cluster, follow these additional steps:

1. Configure Red Hat HA to add the disk and IP resources associated with the second instance of Caché.

2. Bring the new resources online so the disks are mounted on one of the nodes.

3. Be sure the users and groups to be associated with the new instance are created and synchronized between nodes.

4. On the node with the mounted disk, run ccinstall following the procedures outlined in the "Installing Caché on UNIX® and Linux" chapter of the Caché Installation Guide.

5. Stop Caché.

6. Synchronize the Caché registry using the following steps:

   1. On the install node run

      ccontrol list
      

      

   2. Record the instance name, version ID and installation directory of the instance you just installed.

   3. On the other node, run the following command to create the registry entry, using the information you recorded from the recently installed instance:

      ccontrol create instname versionid=vers_ID directory=installdir
      

      

7. Add the Caché resource for this instance to your cluster as follows:

   pcs resource create instname ocf:heartbeat:Cache Instance=instname cleanstop=1
   

   

8. The new Caché resource must be configured to collocate and ordered to start after its disk resources and optionally its VIP resource(s). To prevent unexpected stops and restarts, Caché and its collocated resources should be configured to prefer their current location (resource-stickiness=INFINITY) and thus never fail back after a node reboots.

9. Verify that Caché starts in the cluster.

Failure Testing

Typical full scale testing must go beyond a controlled service relocation. While service relocation testing is necessary to validate that the package configuration and the service scripts are all functioning properly, you should also test responses to simulated failures. Be sure to test failures such as:

• Loss of public and private network connectivity to the active node

• Loss of disk connectivity

• Hard crash of active node

Testing should include a simulated or real application load. Testing with an application load builds confidence that the application will recover in the event of actual failure.

If possible, test with a heavy disk write load; during heavy disk writes the database is at its most vulnerable. Caché handles all recovery automatically using its CACHE.WIJ and journal files, but testing a crash during an active disk write ensures that all file system and disk devices are properly failing over.

Software and Firmware Updates

Keep software patches and firmware revisions up to date. Avoid known problems by adhering to a patch and update schedule.

Monitor Logs

Keep an eye on the /var/log/pacemaker.log file and messages file in /var/log/ as well as the Caché cconsole.log files. The Caché agent resource script logs time-stamped information to the logs during cluster events.

Use the Caché console log, the Caché Monitor and the Caché System Monitor to be alerted to problems with the database that may not be caught by the cluster software. (See the chapters “Monitoring Caché Using the Management Portal”, “Using the Caché Monitor” and “Using the Caché System Monitor” in the Caché Monitoring Guide for information about these tools.)