Installing applications on the cluster. Installing an application in a cluster setting requires planning. As I mentioned, avoid applications that update the Registry during operation. Identify the configuration files, status files, and log files that the application reads, updates, and writes. You need to store these files on cluster disks so that the files will move with the application from one server to the other.
Determine the application's resource dependency relationships. For example, you can't bring a database online before its data disks and client-access network name, and the network name depends on the associated virtual server's IP address. You can define dependency relationships between resources in the same resource group only; therefore, all resources that you associate with an application must be in the same group as the application. If multiple applications or multiple application instances (e.g., databases) share a resource, all the applications or instances must be in the same group. This requirement restricts your ability to load-balance different applications between two servers. When you want to run the same application on both servers, you need to define the application's resources on both disks as well. If the application uses any Cluster Group resources (e.g., the cluster network name), you'll need to add the application's resources to the Cluster Group. To install and validate a typical application or instance, follow these steps:
- Create a resource group for the application. Define appropriate cluster disks in the group. Define other relevant resources (e.g., network name, virtual server IP address). Confirm that you can bring the group online from both servers.
- Bring the group online on one server (i.e., server A). Install the application on server A, and configure the application to use the cluster storage. Confirm that the application functions correctly (i.e., starts, runs, and stops correctly) on server A.
- Define the application services or programs as cluster resources. Set services to start manually rather than automatically. Confirm that you can use Cluster Administrator to correctly stop and start the application.
- Move the group to the other server (i.e., server B). Install the application on server B and configure the application to use the cluster storage; take care not to overwrite any settings that you defined on the cluster storage when you configured the application on server A. Again, set any services to start manually. Confirm that you can use Cluster Administrator to correctly stop and start the application on server B.
- Confirm that the application will fail over from one server to the other in all circumstances (e.g., manual server shutdown and server failure).
- Follow the same process for other applications. After each installation, confirm that the existing applications still work correctly.
You can configure utility software, such as backup or system-monitoring software, to recognize each server individually or to recognize the cluster environment as a whole. The correct approach depends on the utility's characteristics and functions. For example, suppose you want to use a simple system-monitoring utility to monitor a clustered service. The utility sends SNMP alerts to a management framework when certain services aren't running. But at any time, the clustered service is running on one server and stopped on the other. If you run the monitoring utility on both servers, the monitor on the server with the stopped service won't be aware that the service is running correctly on the other server and will constantly generate alerts.
One solution might be to include in the management framework logic that recognizes that the two servers are related and that only raises an alert when both servers report that the service isn't running. Another solution might be to integrate the system monitor into the cluster with one application instance running and use the MSCS cluster command-line utility to confirm that the monitored service is online.
Maintenance
You've created a stable, highly available environment—now you want to keep it that way. And if something goes wrong, you need to be able to put it right quickly.
If you need to perform hardware or software maintenance on a clustered server, you can move all the server's resource groups to the other server and perform the maintenance while the application services are online. However, the best approach is to perform the work during application downtime, if possible. That way, you can avoid unexpected service loss if something goes wrong. In particular, don't follow the rolling service pack upgrade method that Microsoft describes in recent service pack release notes; wait until you can upgrade both servers while the application services are offline.
MSCS doesn't have any Distributed Lock Manager (DLM) functions; therefore, only one server at a time can access a particular disk. By default, the MSCS software grants cluster-disk access to a server only when Cluster Service determines that server as the rightful owner of that disk. Therefore, Cluster Service must start successfully on a server and either form or join a cluster before the server can access cluster disks. If a disk problem (e.g., a corrupt quorum log) causes Cluster Service to fail, you can't access the disk to repair the problem. If the quorum log becomes corrupt, Cluster Service writes an error identifying the problem to the NT System log before the service shuts down. You can use the noquorumlogging parameter to manually start Cluster Service from the Control Panel Services applet. This parameter lets Cluster Service start without trying to open the quorum log. Delete the quorum log (i.e., \mscs\quolog.log on the quorum disk), then stop and restart Cluster Service. The service will create a new quorum log.
| MSCS RESOURCES |
BOOKS
Windows NT Microsoft Cluster Server
Author: Richard Lee
Publisher: (McGraw Hill, 1999)
|
|
Microsoft Articles |
"Cluster Server Troubleshooting and Maintenance White Paper"
http://support.microsoft.com/support/ kb/articles/q238/6/27.asp
|
|
Related Articles in Previous Issues |
You can obtain the following articles from Windows 2000 Magazine's Web site at http://www.win2000mag.com/articles.
BRAD COOPER
"Installing Microsoft Cluster Server," October 1998 Web Exclusive, InstantDoc ID 3923
JONATHAN CRAGLE
"Balanced Cluster Service," February 1999, InstantDoc ID 4812
RON MILIONE
"Paving the Way for Microsoft Cluster Server," May 2000 Web Exclusive, InstantDoc ID 8207
JIM PLAS
"Build a High-Availability Web Site with MSCS and IIS 4.0," June 1999, InstantDoc ID 5371
MARK RUSSINOVICH
NT Internals, "Inside Microsoft Cluster Server," February 1998, InstantDoc ID 2943
BARRIE SOSINSKY
"NT Clusters," November 1999, InstantDoc ID 7291 |
As another complication, you can't run Chkdsk on a cluster disk. Cluster Service locks cluster disks, so Chkdsk can't access them, and cluster disks come online only after Cluster Service starts, so you can't set Chkdsk to successfully run during a reboot. If you want to run Chkdsk on a cluster disk, shut down one server and use the other server's emergency recovery installation to boot the first server. Because the recovery installation doesn't include cluster software, it doesn't restrict access to the cluster disks. You can now run Chkdsk on the problem disk, then reboot both servers through the regular cluster installation.
Some software, drivers, and utilities might not be perfectly suited to a clustered environment. Before you modify a cluster, back up the system and refresh the ERD. Make the change, then confirm that all cluster operations still function and resource groups still move between servers. Backup procedures can be complicated because you can connect drives to either cluster server. Develop a simple procedure that will work under typical conditions, then develop specific procedures for specific failure scenarios.
MSCS or Bust?
MSCS can let you have NT and high availability at a reasonable cost, but you need to consider your business needs, applications, and hardware before deciding whether MSCS is the right tool for your environment. The effort you put into cluster installation and management will affect your cluster's success—and reflect the importance your organization places on its services.
Register
