SAP with Microsoft SQL Server 2005: Best Practices for High Availability, Maximum Performance, and Scalability18
Database mirroring is a new feature in SQL Server 2005. Database mirroring will become available for use in the first half of 2006. At the time of this writing, only a small number of selected customers are using database mirroring in a production environment.
Database mirroring ensures that a transactionally consistent, hot-standby mySAP database is rapidly available in the event of a database failure. Database mirroring allows for geographical redundancy with network traffic encrypted by default.
SQL Server 2005 offers three database-mirroring configurations:
Synchronous mirroring with failover
Synchronous mirroring without failover
The appropriate configuration to use with SAP depends primarily on the customer’s specific failover requirements and infrastructure. However, synchronous mirroring without failover is not recommended for SAP systems.
Asynchronous mirroring and synchronous mirroring with failover are described in the following sections.
Asynchronous database mirroring uses a standby server when network resources or physical distance precludes the use of synchronous mirroring with failover. The transaction log records are sent asynchronously to the mirrored server. The primary database server does not wait for the mirrored server to acknowledge the receipt of the transactions before confirming the commit to the application.
Asynchronous mirroring does not guarantee that all transactions committed on the principal server will be saved to the mirrored server. With asynchronous mirroring, it is not possible to have automatic failover. However, the secondary system is available in the event of a failure.
Asynchronous mirroring can be used in disaster-recovery scenarios when the database is mirrored to a remote site across wider distances. Currently, most SAP customers use log shipping for that purpose. Asynchronous mirroring is an effective alternative that improves transactional consistency and, most importantly, provides a mirrored image of the database that is close to the principal image of the database.
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