Q: Why would I use Double-Take GeoCluster instead of setting up MSCS with just a shared disk? Get Answer
A:
The shared disk subsystem and storage path (SCSI or Fibre Channel) represents a single point of failure for the entire cluster. If the power goes out on the subsystem or someone knocks the SCSI cable out, the cluster will go down. Both the cluster nodes and the shared storage are vulnerable to geographic disasters such as fires, flood, earthquakes or hurricanes. Since Double-Take GeoCluster replicates multiple copies of the data over standard network links using TCP/IP, the cluster can be designed to protect against geographic disasters.
Microsoft support for MSCS and MSCS-based Microsoft applications requires that the exact cluster configuration appear on the Microsoft Hardware Compatibility List under category "Cluster". This means you typically cannot take a server you have running and decide to cluster it using MSCS alone.
Given the hardware nature of MSCS clustering and the cluster certification tests, it is not currently possible for NSI Software to submit a GeoCluster software based MSCS configuration to Microsoft for certification because the tests are designed to test hardware. While not supported by Microsoft, because it is independent of hardware, GeoCluster can be installed with MSCS on any 2000/NT supported server and only requires that disks exist with matching drive letters and the same amount of disk space on both nodes of the cluster. Even servers of from different vendors can be successfully clustered. Keep in mind that application failover using other 3rd party failover technologies would not be considered "supported" by Microsoft either and are supported by the vendor that provides the solution as is the case with GeoCluster.
NSI is working with Microsoft to modify the cluster certification tests to allow GeoCluster based configurations to be added to the Microsoft Hardware Compatibility List for Clusters in the future.
Q: Standard Double-Take for Windows 2000/NT runs on every 4.0 and later version of NT from Workstation to Terminal Server, what about GeoCluster? Get Answer
A: Because GeoCluster is an extension of Microsoft Cluster Service, it can only be used with operating systems that include this feature. This currently includes Windows NT Server 4.0. Enterprise Edition and Windows 2000 Advanced Server or Datacenter. Because of this, GeoCluster cannot be used on standard Windows NT Server (non- Enterprise Edition), Windows NT Terminal Server Edition, Windows NT Workstation, Windows 2000 Professional or Windows 2000 Server.
Q: Does GeoCluster support clustering multiple instances of SQL Server 7.0 on a cluster? Get Answer
A: GeoCluster supports the same configurations of SQL Server 7 Enterprise edition as MSCS allows on a shared disk configuration. Multiple SQL virtual servers can be created in multiple cluster groups and moved independently between each node.
Q: How far apart can the nodes be? Are there any limitations on the network configurations allowed? Get Answer
A: There is no inherent distance limitation between the nodes. Currently, the network connections between each node must be on the same IP subnet. For example, each node may have both a private network connection (192.168.1.1 and 192.168.1.2 on 255.255.255.0) and a public connection for user access (10.0.0.1 and 10.0.0.2 on 255.0.0.0). The NICs must be on the same IP subnet so that the IP addresses can be hosted on either node, and because the cluster heartbeat messages are UDP packets and will not cross routers. This configuration could be established across great distances using VLAN (virtual LAN) and VPN (Virtual Private Network) capabilities in many modern networking devices. Future versions of GeoCluster may alleviate this restriction or provide additional alternatives. Currently, for configurations beyond 20 km we recommend advanced consulting with NSI Software engineers.
