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11.3.2 Mirroring restrictions
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Before we continue, let s cover some of the restrictions of database mirroring:
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Unlike transaction log shipping, which supports the Bulk_Logged recovery model, database mirroring only works for databases using the full recovery model. The mirror database can t be read unless you create a database snapshot against it. We covered database snapshots in the previous chapter. Depending on the environment and mirroring mode, a snapshot may introduce an unacceptable performance overhead. Mirroring is set up for individual databases in other words, you can t mirror at an instance level. While you can individually mirror more than one database within an instance, there are special considerations when mirroring databases with interdependencies, a topic we ll cover later in this chapter. System databases (master, model, msdb, and tempdb) can t be mirrored.
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High availability with database mirroring
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You can have only one mirror database for each principal, unlike log shipping where logs from a source database can be shipped to multiple destinations. However, the one database can be both log shipped and mirrored. Databases involved in cross-database or distributed transactions aren t supported in database mirroring. We ll explore this restriction in more detail later in the chapter. As covered in chapter 9, database mirroring can t be enabled on databases containing FileStream data.
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With that brief introduction in mind, let s move on and discuss the two major types of database mirroring modes: asynchronous (high performance) and synchronous (high safety).
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11.4 Mirroring modes
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Later in the chapter we ll go through the process of preparing for and setting up a mirroring session. For now, let s concentrate on the process behind the delivery of transactions from the principal to the mirror during a mirroring session, and how this process is performed based on the mirroring mode. A database mirroring session begins with the mirror instance identifying the log sequence number (LSN) of the last transaction applied to the mirror database. The mirror instance then obtains any outstanding transactions from the transaction log of the principal database. The outstanding transactions received from the principal instance are written to the transaction log of the mirror database and rolled forward. The outstanding transactions to roll forward are known as the redo queue, and the depth of this queue determines the catch-up time and therefore the minimum time to fail over the principal role to the mirror database. The mirroring process for a synchronous mirroring session is summarized in figure 11.9.
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Synchronous Database Mirroring (High Safety) Principal DB Mirroring DB
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1) Transaction written to log 2) Log record streamed to mirror 3) Log record hardened to transaction log 4) Transaction commits on the principal 5) Transaction rolled forward on the mirror DB
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Figure 11.9 Synchronous database mirroring. In asynchronous mirroring, the transaction commits on the principal database after step 2.
Mirroring modes
As updates on the principal database continue, the transactions are streamed from the principal s transaction log to the mirror s transaction log and rolled forward on the mirror database. The mirroring mode, asynchronous (high performance) or synchronous (high safety), determines how the principal s transactions are sent and received.
11.4.1 High performance (asynchronous)
Asynchronous mirroring is only available in the Enterprise edition of SQL Server. Under asynchronous mode, a transaction is committed on the principal as soon as it s sent to the mirror; it doesn t wait for an acknowledgment from the mirror that the transaction has been written to the mirror s transaction log, nor is the principal affected in any way by a failure at the mirror (other than a loss of failover capabilities). As such, asynchronous mirroring is used when transaction performance at the principal is of prime concern. The high-performance nature of asynchronous mode comes with a reduction in high availability. In cases where the transaction load at the principal is very high, or the mirror server is overloaded (or both), the redo queue on the mirror may become very deep, increasing failover time. Further, given the transaction delivery method, there s no guarantee that the mirror partner receives and applies each transaction.
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