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A database can become inaccessible because of a runaway transaction or other operations. How ever, Database Mirroring does not detect these as failures; only a failure of the ping test is consid ered a failure. You also have to carefully balance the number of mirroring sessions that are configured on a single instance. Each database participating in a mirroring session must be pinged every couple of sec onds to determine whether it is still connected and available. The witness must also be pinged by both the principal and the mirror every couple of seconds to determine whether it is still connected and available. The failure of a ping test causes a failover in High Availability operating mode. Hav ing a large number of databases participating in mirroring sessions on a single server increases the possibility of an anomalous failover because of saturation of the network.
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When the Database Mirroring session fails over, SQL Server reverses the roles of the
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principal and mirror. SQL Server promotes the mirror database to the principal and
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begins serving the database; it then demotes the principal database to the mirror. SQL Server also automatically reverses the transaction flow. This process is a significant improvement over other availability methods such as replication or log shipping, which require manual intervention or even reconfiguration to reverse the transaction flow. In this automatic failover process, the mirror essentially promotes itself to principal and begins serving the database. But first, the witness server must arbitrate the failover and role reversal by requiring two of the three Database Mirroring roles or a quorum to agree on the promotion. A quorum is necessary to prevent the database from being served on more than one instance within the Database Mirroring session. If the principal were to fail and the mirror could not connect to the witness, it would be impossible to reach a quorum, and SQL Server would then not promote the mirror to the principal.
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If the mirror were allowed to determine that it should serve the database by itself, it could intro duce a situation in which the database would be accessible to transactions on more than one server. This is referred to as a split-brain problem.
High Availability operating mode s automatic failure detection and failover follow these general steps: 1. The principal and mirror continuously ping each other. 2. The witness periodically pings both principal and mirror. 3. The principal fails. 4. The mirror detects the failure and makes a request to the witness to promote itself to the principal database. 5. The witness cannot ping the principal but can ping the mirror, so the witness agrees with the role reversal, and SQL Server promotes the mirror to the principal. 6. The principal server comes back online from the failure and detects that the mir ror has been promoted to principal.
Lesson 1: Overview of Database Mirroring
7. SQL Server demotes the original principal to a mirror, and transactions begin flowing to this database to resynchronize it with the new principal.
IMPORTANT
Hot standby: witness must be online
Automatic failover can occur only if the witness server is online. If the witness is offline, there is no automatic failover. This means that you can use High Availability operating mode to provide a hot standby server only when the witness server is online. Otherwise, you have a warm standby configuration.
High Performance Operating Mode
High Performance operating mode uses a principal and a mirror database, but does not need a witness server. This operating mode provides a warm standby configuration that does not support automatic failure detection or automatic failover. High Performance operating mode does not automatically fail over because transac tions are sent to the mirror asynchronously. Transactions are committed to the prin cipal database and acknowledged to the application. A separate process constantly sends those transactions to the mirror, which introduces latency into the process. This latency prevents a Database Mirroring session from automatically failing over because the process cannot guarantee that the mirror has received all transactions when a failure occurs. Because the transfer is asynchronous, High Performance operating mode does not affect application performance, and you can have greater geographic separation between the principal and mirror. However, this mode increases latency and can lead to greater data loss in the event of a primary database failure.
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