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Microsoft® SQL Server 2012 Unleashed by Alex T. Silverstein, Chris Gallelli, Paul T. Bertucci, Ray Rankins

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Copy-on-Write Technology

The copy-on-write technology is at the core of the database mirroring capability. Look back at Figure 41.1, and notice what happens in a High Safety with Automatic Failover mode (synchronous mode):

1. A transaction from a client connection to the principal server (A) is written to the AdventureWorks database (D).

2. When the transaction is written to the principal server’s transaction log, it is immediately copied (B) and written to the mirror server (D).

3. When this physical log record is written to the mirror server, it sends back an acknowledgment (C) to the principal of its write success.

This is the copy-on-write technology. The end result is that the mirror server is in exactly the same state as the principal server ...

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