Microsoft® SQL Server® 2012 Bible

692

Part V: Enterprise Data Management

performance, but a heavy load on the principal database or a network delay could cause

the principal database transaction log waiting to be sent to the mirror database to grow.

In the event of a failure of the principal, the unsent transaction log records may be lost.

Automatic and manual failover is not enabled in high-performance mode due to possible

data loss. Only a forced failover is allowed in this mode.

FIGURE 27-2

Asynchronous database mirroring mode.

1

3

2

5

2 >2 4 >4

Client

Principal Server

SQL Server SQL Server

Log DB Log DB

Mirror Server

Mirror is busy

keeping up with

the principal

Asynchronous database mirroring (high-performance mode) is only supported by SQL Server 2012 Enterprise or

Developer Editions.

Best Practice

Do not confi gure a witness server in high-performance operating mode because there is no use of a

witness server. If you use a witness server and for some reason the witness server and mirror server

both become unavailable, the principal server cannot form a quorum, and the principal database is not

accessible to the clients, even though the principal server is available and does not have any issues.

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