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Professional Microsoft® SQL Server® 2008 Administration by Steven Wort, Ross LoForte, Wayne Snyder, Ketan Patel, Brian Knight

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17.4. Preparing the Mirror Server for Failover

When you set up mirroring, your intentions are clear that in the event of failover, the mirror takes on the full load. In order for the mirror to be fully functional as a principal, you have to do some configurations on the mirror server. Please note that database mirroring is a database-to-database failover solution only; the entire SQL Server instance is not mirrored. If you want to implement full SQL Server instance failover, then consider Windows failover clustering. We compare different high-availability technologies in the section "Database Mirroring and Other High-Availability Solutions."

17.4.1. Hardware, Software, and Server Configuration

Your mirror server hardware should be identical (CPU, memory, storage, and network capacity) to that of the principal if you want your mirror server to handle the same load as your principal. You may argue that if your principal is a 16-way, 64-bit server with 32GB of RAM, having identical mirror hardware is a costly solution if your principal is not going down often and such expensive hardware is sitting idle. If your application is not critical, then you may want to have a smaller server just for failover and taking the load for some time, but I would argue that if the application is not that critical, you do not have such extensive hardware on the primary either.

Moreover, with such huge hardware, the process on the server must be very heavy, so if you failover, your mirror should be ...

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