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10: High-Availability Strategies
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Behind the fa ade of this virtual server, two nodes exist We could actually have as many as eight nodes, depending on the operating system and SQL Server versions The nodes are running fully functional operating systems with SQL Server However, the databases themselves are contained on a disk subsystem available to both Since the disk subsystem becomes a single point of failure, it is often protected with a hardware RAID or SAN We wouldn t invest tens of thousands of dollars in a second server and the extra hardware required for a cluster, only to leave the data on a single drive Notice the heart between the two servers It s not because they re in love Both servers typically have a separate NIC connected to the other; they constantly monitor the heartbeat of the other server This is how the inactive node identifies that the primary node has failed Often it will try to resuscitate the primary node, perhaps by restarting services If the primary node doesn t come back, the secondary node takes over
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When designing a cluster, a first step is identifying how many nodes to configure Most current cluster configurations require that at least one node be inactive This gives us a simple formula of N + 1 for the number of nodes, where N is the number of primary servers we will support Take a look at Figure 10-31 This shows a cluster with two active nodes and one inactive Each active node in the cluster is associated with a virtual server and an actual IP address If an active node fails and the cluster fails over to the inactive node, the cluster will automatically route requests to the new active node
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Figure 10-31 A three-node cluster
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Virtual server 1 Single IP Data Node 2 Active
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Virtual server 2 Single IP Node 3 Inactive Node 3 can become active for Node 1 or 2
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It s not uncommon to have two inactive nodes in clusters with as many as eight nodes Having two inactive nodes allows two of the six active nodes to fail, and each of the inactive nodes can take up the slack Can we use just one inactive node with many active nodes Yes, but we need to make sure that we ve calculated the load Consider Table 10-2 If all eight servers were to have identical hardware, then SQL8 could pick up the load if any one of the servers were to fail However, if SQ1 and SQL3 both failed at the same time, SQL8 simply couldn t pick up the load for SQL 1 (at 65% utilization) and SQL 3 (at 60% utilization) It d be pegged at 100% processor utilization This scenario is an ideal situation where a 6 + 2 cluster would be created
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Server Processor utilization
SQL1 SQL2 SQL3 SQL4 SQL5 SQL6 SQL7 SQL8
65% 35% 60% 55% 40% 45% 75% Inactive
Table 10-2 Processor Utilization on Different Nodes
While we covered how many nodes are supported in SQL Server editions and Windows Server editions in 1, let s remind ourselves here as they relate to clustering Table 10-3 shows exactly how many nodes are supported by the different editions
Edition Number of nodes
SQL Server Standard Edition SQL Server Enterprise Edition Windows Server 2003 Enterprise Windows Server 2003 DataCenter
2 8 8 8
Table 10-3 Number of Cluster Nodes Supported in Different Editions
EXAM TIP Make sure you know how many cluster nodes are supported on the different editions of SQL Server and the different editions of Windows Server 2003 SQL Server 2005 Standard Edition supports two nodes SQL Server 2005 Enterprise Edition supports eight nodes Windows Server 2003 Enterprise and DataCenter editions support as many as eight nodes in clusters
10: High-Availability Strategies
Cost vs Benefits
Since clusters provide automatic failover and can provide an almost-always-on capability, why not just use them all the time Cost Clusters require substantially higher cost to implement Even though you have one or more of the nodes inactive, you still have to purchase the hardware and licenses for the software This is sometimes a hard sell to management when you re asking for $20,000 for a server that will rarely be used The cost of the disk hardware can be a substantial extra cost depending on the system purchased Additionally, clusters are difficult to configure and maintain From a total cost of ownership (TCO) perspective, clusters add significantly to the mix If you ve never set up a cluster before, believe me, the first time is rarely smooth What s an alternative Microsoft wants you to know that database mirroring is a cheaper software-based solution While database mirroring doesn t back up the entire server, generally when we need a cluster, it s because we have one or two very large databases, not hundreds When database mirroring is configured with a witness in synchronous mode, automatic failover can be achieved When configured with a database snapshot, concurrent use is even possible
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