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Multiple Mirrors

By Steve Jones,

publishing model

Someone asked the question the other day if they could miror their database to two locaitons. As in log shipping. They had a primary server, and then a local failover, but wanted to implement a second, emergency failover in another location.

I wasn't sure, and didn't see it documented explicitly in a few places, so I asked the question and got a definitive answer from Paul Randal, who pointed me to this white paper (.DOC) outlining how Log Shipping and Mirroring work together. However should we evolve mirroring to work differently? Would the publisher model make sense here? Is there some value in having a separate mirror server setup on a lower priority that can act as a tertiary failover point?

I am torn on this point because I think mirroring is fairly simple, but it seems to be complex to quite a few people, and there are enough reports of issues in failover/failback scenarios that it seems to problematic as a DR/HA solution. Adding in additional configuration in terms of priority for failover, and failback, as well as a more complicated witness configuration might not help speed adoption.

On the other hand, having the log entries be written to some type of distributor and then farmed out to other systems, seems to be a model that could provide greater capabilities for maintaining your system than we have ever had. Imagine being able to send log entries to some "mirroring distributor" on another machine in high safety mode. They could recover a local failover database on that same server with some two phase commit, but then send those same transactions in high performance mode to a remote DR server, possibly across a lower bandwidth connection. We could even possibly have some commands, like index rebuilds, stripped out for that lower priority mirror.

What about the possibility of even having those log entries replayed on a "live" reporting database, similar to replication, that is accessible by users?

Ultimately I come down on the side that we ought to develop and build valuable enhancements into SQL Server and force people to become more skilled to implement them. Running a SQL Server is easier than other platforms, but it's not necessarily easy. A certain level of skill is still required.

Steve Jones

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Today's podcast features music by Everyday Jones. No relation, but I stumbled on to them and really like the music. Support this great duo at www.everydayjones.com.

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