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Configuring failover monitoring

Before beginning your failover configuration, review your IP address and subnet configuration on the source. Because of limitations in the way the Linux kernel handles IP address aliases, you will not be able to mix subnets on the eth0 network interface. Failover should not cause problems in this configuration, but you will lose IP addresses during failback. Therefore, if you must mix subnets on a single interface, use eth1 or higher.

1. The Failover Control Center can be started from within the Replication Console or from the Windows desktop.
• From the Replication Console, select Tools, Failover Control Center.
• From the Windows desktop, select Start, Programs, Double-Take for Linux, Availability, Double-Take Failover Control Center.

2. Select a failover target from the Target Machine list box.

   If the target you need is not listed, click Add Target and manually enter a name or IP address (with or without a port number). You can also select the Browse button to search for a target machine name. Click OK to select the target machine and return to the Failover Control Center main window.

    

3. Click Login to login to the selected target.
4. Select a source machine to monitor by clicking Add Monitor. The Insert Source Machine dialog box appears in front of the Monitor Settings dialog box.

5. On the Insert Source Machine dialog, specify your source machine by either of the following methods.

   

   • Type the name of the machine that you want to monitor in Machine Name(s) and click OK.
   • Click Custom. Enter the name of the server and click Add. Specify the IP address and subnet mask of the specified server and click OK. Click OK again.

   The Insert Source Machine dialog closes and the Monitor Settings dialog remains open with your source listed in the Names to Monitor tree.

6. In the Names to Monitor tree, locate and select the IP addresses on the source that you want to monitor.

7. Highlight an IP address that you have selected for monitoring and select a Target Adapter that will assume that IP address during failover. Repeat this process for each IP address that is being monitored.

   Current IP Addresses displays the IP address(es) currently assigned to the selected target adapter.

    

8. Highlight an IP address that you have selected for monitoring and select a Monitor Interval. This setting identifies the number of seconds between the monitor requests sent from the target to the source to determine if the source is online. Repeat this step for each IP address that is being monitored.

9. Highlight an IP address that you have selected for monitoring and select the Missed Packets. This setting is the number of monitor replies sent from the source to the target that can be missed before assuming the source machine has failed. Repeat this step for each IP address that is being monitored.

   To achieve shorter delays before failover, use lower Monitor Interval and Missed Packets values. This may be necessary for IP addresses on machines, such as a web server or order processing database, which must remain available and responsive at all times. Lower values should be used where redundant interfaces and high-speed, reliable network links are available to prevent the false detection of failure. If the hardware does not support reliable communications, lower values can lead to premature failover. To achieve longer delays before failover, choose higher values. This may be necessary for IP addresses on slower networks or on a server that is not transaction critical. For example, failover would not be necessary in the case of a server restart.

    

   

10. Highlight the source name and specify the Items to Failover, which identifies which source components you want to failover to the target.
• IP Addresses—If you want to failover the IP addresses on the source, enable this option and then specify the addresses that you want to failover. When the source and target are on the same subnet, generally a LAN environment, you should failover the IP address. If the source and target are on different subnets, generally a WAN environment, you should not failover the IP address. See WAN considerations for options on handling WAN failover.
• Monitored only—Only the IP address(es) that are selected for monitoring will be failed over.

• Include Unmonitored—All of the IP address(es) will be failed over.

  If you are monitoring multiple IP addresses, IP address conflicts may occur during failover when the number of IP addresses that trigger failover is less than the number of IP addresses that are assumed by the target during failover. For example, if a source has four IP addresses (three public and one private), and two of the three public addresses are monitored, but all three public addresses are configured to failover, a conflict could occur. If the source fails, there is no conflict because all of the IP addresses have failed and no longer exist. But if the failure only occurs on one of the monitored addresses, the other two IP addresses are still affected. If all of the addresses are failed over, these addresses then exist on both the source and the target. Therefore, when a source machine has fewer IP addresses that trigger failover than IP addresses that will be failed over, there is a risk of an IP address conflict.

   

11. By default, Manual Intervention is enabled, allowing you to control when failover occurs. When a failure occurs, a prompt appears in the Failover Control Center and waits for you to manually initiate the failover process. Disable this option only if you want failover to occur immediately when a failure occurs.
12. If you are using any failover or failback scripts, click Scripts and enter the path and filename for each script type. Scripts may contain any valid Linux command, executable, or script file. Examples of functions specified in scripts include stopping daemons on the target before failover because they may not be necessary while the target is standing in for the source, stopping daemons on the target that need to be restarted with the source’s machine name and IP address, starting daemons or loading applications that are in an idle, standby mode waiting for failover to occur, notifying the administrator before and after failover or failback occurs, stopping daemons on the target after failback because they are no longer needed, stopping daemons on the target that need to be restarted with the target machine’s original name and IP address, and so on.Specify each script that you want to run and the following options, if necessary.
13. If you want to delay the failover or failback processes until the associated script has completed, mark the appropriate check box.
14. If you want the same scripts to be used as the default for future monitor sessions, mark the appropriate check box.
15. Click OK to return to the Monitor Settings dialog box.
16. Click OK on the Monitor Settings dialog box to save your monitor settings and begin monitoring for a failure.

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