Is it possible to install Microsoft SQL Server in active/active (symmetric) configuration on VERITAS Cluster Server?
In the context of clustering technologies, the term "active/active" has two interpretations: parallelism, and running multiple instances of Microsoft SQL Server on the same node.
In parallel implementation, the cluster's shared storage is partitioned across multiple cluster nodes, which access the data simultaneously. Synchronization of tasks with serialized dependencies ensures that transactions are completed in the correct order, and locking mechanisms are used to avoid conflicted use of the same resource by different tasks. In the case of a node failure, the node's workload is failed over to another node in the cluster.
2. Multiple Instances
This implementation involves the deployment of multiple instances of Microsoft SQL Server on the same node. Each node in a cluster runs an instance of SQL so that no server is left idle. If one node fails, its instance of SQL is failed over to another node (already running its own SQL instance), thereby resulting in multiple SQL instances on a single node.
VERITAS Cluster Server for Windows NT Parallelism VERITAS Cluster Server for Windows NT does not support parallelism due to the nature of the underlying operating system. Limitations of the NTFS file structure, specifically its inability to avoid conflict through a file locking mechanism, render this configuration impossible in any clustering technology that relies on NTFS.
Multiple Instances VERITAS Cluster Server for Windows NT does not support the deployment of multiple instances of Microsoft SQL Server 7.0 on a single system. However, this concept of active/active necessarily results in a large percentage of wasted resources: in normal operation of a two-node cluster, each node must run at only 50% of its total load so that enough capacity is available for takeover should the other node fail.
N-to-1 Cluster Server offers a concept called N-to-1, whereby a single standby server is available to take over for any one of multiple nodes running SQL. This configuration is based upon the concept that multiple, simultaneous server failures are unlikely, and therefore a single backup server can protect multiple active servers. For example, in a 4-to-1 configuration, one server protects four other servers, thereby reducing redundancy costs at the server level from 100% to 25%.
N-to-N Cluster Server also supports N-to-N configuration, whereby multiple service groups are configured on multiple servers. Each service group is capable of being failed over to different servers in the cluster. For example, each node of a 4-node cluster is configured with three instances of a critical database. If any node fails, each of its three instances is failed over to a different node, so ensuring that no single node becomes overloaded. This configuration provides for cluster standby
capacity rather than an entire standby server.
VERITAS Cluster Server for Windows 2000 Parallelism VERITAS Cluster Server for Windows 2000 does not support parallelism due to limitations of the NTFS file system, as described above.
Multiple Instances As well as supporting N-to-1 and N-to-N configurations, VERITAS Cluster Server for Windows 2000 supports up to 16 instances of Microsoft SQL Server 2000 on a single node. Any SQL Server 2000 instance can failover to any other configured node.
Note: Group failovers must be configured so that no one node ever hosts more than 16 instances after failover. Even with one instance per node, this can be an issue because Cluster Server allows up to 32 nodes in a cluster.
See the VERITAS Cluster Server 2.0 Enterprise Agent for SQL Server 2000 Installation and Configuration Guide for more information on configuring multiple instances of Microsoft SQL 2000
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