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-- Resume Mirroring ALTER DATABASE Sales SET PARTNER RESUME
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Once resumed, the mirroring session will enter the synchronizing state, with the mirroring database catching up on transactions that have occurred since mirroring was suspended. In addition to pausing and resuming the mirroring session, we can also initiate failover.
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11.6.4 Initiating failover
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Manually initiating failover is possible in either high-safety mode (synchronous mirroring) through manual failover, or in high-performance mode (asynchronous mirroring) using forced service. When the mirroring databases are in the synchronized state, manual failover is specified by running the ALTER DATABASE SET PARTNER FAILOVER command on the principal database, as in this example:
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-- Initiate Manual Failover ALTER DATABASE Sales SET PARTNER FAILOVER
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At this point, clients are disconnected, active transactions are rolled back, and the roles of principal and mirror are swapped. As mentioned earlier, the only failover mode supported for databases mirrored in high-performance mode is the forced service option, which may result in data loss. The forced service option is executed on
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Mirroring in action
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the mirror database as shown in the following example, and is only available when the principal instance is disconnected:
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-- Force Failover - run on mirror instance - data loss possible ALTER DATABASE Sales SET PARTNER FORCE_SERVICE_ALLOW_DATA_LOSS
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In addition to the T-SQL methods, we can use Management Studio to implement failover mirroring using the Mirroring page of a database s properties window, as shown earlier in figure 11.11. Before concluding the chapter, let s cover some important considerations when mirroring multiple databases on the same SQL Server instance.
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11.6.5 Considerations for mirroring multiple databases
One of the limitations with a database mirroring session is that it s used to mirror a single database with other mirrored databases on the server operating on different mirroring sessions and therefore failing over independently. One of the implications of this for connected databases is that if there are interdependencies between two or more mirrored databases, there s no way of ensuring that they fail over as a group. Take, for example, an application that uses two or more databases. If each of the databases can fail over independently of the other, and the application uses a single connection string for the server and instance name, problems will result when only one database fails over. In most cases, a failure will occur at an instance level, causing all mirrored databases to fail over at once, but individual disk failures or temporary network problems may cause single database failovers. To reduce the problems associated with multiple mirrored databases on the same instance, consider these practices:
Configure all principal databases to fail to the same mirror instance. This ensures shared connection settings can use the one failover instance for multiple databases. Set up alerts, covered in chapter 14, for failover events. Such alerts can be used to ensure all databases fail together, and potentially can be used to automate the manual failover of remaining databases. Consider alternate high-availability strategies. Depending on the situation, failover clustering may present a better alternative to database mirroring.
There s one final thing to consider for mirroring multiple databases on the same instance: additional worker threads, memory, and other resources are used for each mirroring session. The more mirrored databases, the greater the load on the instance, and the higher the network saturation. It follows that each session won t perform as well as the load increases, and in the worst case, may lead to session failovers. There s no maximum number of mirrored databases per instance.2 The load characteristics of each database should be considered in line with the overall server capacity.
Books Online suggests a maximum of 10 mirrored databases for a 32-bit server instance.
High availability with database mirroring
11.7 Best practice considerations: high availability
As with performance tuning where there will always be a bottleneck somewhere, each high-availability plan typically has its own weak points. On its own, database mirroring won t protect against poor maintenance practices and corrupted backups; it should therefore be seen as one component of a much broader high-availability plan.
Service level agreements should be considered your binding contract. You wouldn t pay a builder to build your house without specific instructions and contract conditions; likewise, you can t effectively design and administer a highly available database system without a set of service level agreements in place. Know what they are and have confidence that they re achievable through well-thought-out and simulated disaster-recovery plans. During the process of developing service level agreements, prepare option papers that clearly list alternate options, costs, and corresponding service levels that can be achieved. Such papers assist the decision-making and budgeting process, and set expectations at the appropriate level for both customers and management. To prevent unexpected problems leading to system outages (therefore impacting availability targets), ensure appropriate development and test systems exist to assess the impact of database changes before they reach production; by the time a change makes its way to production, everyone responsible for the change should have complete confidence that it won t have any adverse effects. Database object definition and modification scripts should be stored in a source control tool in order to support reproducing a database at a defined version with options to roll forward changes as required to achieve a desired state. Aside from resource capacity such as the amount of RAM or CPU power, development, test, and production environments should be as identical as possible to minimize unexpected problems. Items that should be the same include collation, CPU platform, SQL version/edition, and service pack/hotfix level. For each production environment, a corresponding load-testing environment should be available and ideally configured identically to production, or at a scale such that the performance differences are well understood. Such environments, in combination with a load-testing tool, are critical in ensuring changes will work under production load. Consider the importance of accurate test data in development/test environments. If possible, provide complete copies of production data, or obfuscate where appropriate for security. Consider cloning a database s histograms and statistics to enable more accurate query execution plans for development databases if production-sized databases can t be used. Use a schema comparison tool such as Visual Studio Team System Database Edition (aka Data Dude) or a third-party tool. These tools are invaluable in troubleshooting schema difference problems.
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