Microsoft® SQL Server® 2012 Bible

691

Chapter 27: Database Mirroring

27

FIGURE 27-1

Synchronous database mirroring mode.

1

5

2

4

2 >2 3 >3

Client

Principal Server

SQL Server SQL Server

Log DB Log DB

Witness Server

Mirror Server

Mirror is always

redoing-it

remains current

Synchronous database mirroring (high-safety mode) is supported by SQL Server 2012 Enterprise, Business

Intelligence, Developer, and Standard Editions. If you implement synchronous database mirroring using SQL Server

2012 Standard Edition, you cannot create database snapshots on the mirror database for reporting purposes

because database snapshots are not supported by the SQL Server 2012 Standard Edition.

Safety off is often referred to as asynchronous database mirroring or high-performance

mode. This safety level provides high performance with possible data loss. In this mode,

the communication between the principal and mirror databases is asynchronous. The

sequence of events when you choose safety off is shown in Figure 27-2. The transaction

log records are written to the principal database transaction log and sent to the mirrored

database transaction log in the same way as in synchronous mirroring mode. The main dif-

ference is that the principal does not wait for the mirror to acknowledge that the transac-

tion has been written to disk. The transactions on the principal database commit as soon

as it is written on the principal database transaction log. This increases the application

c27.indd 691c27.indd 691 7/31/2012 9:50:24 AM7/31/2012 9:50:24 AM

http://www.it-ebooks.info