Up through SQL Server 2008 R2 database mirroring has been limited to a single mirror for each database. While that does work well for many companies, as you become larger and more dependent on your computer systems, there is a need to have multiple mirrors.

Often someone wants to failover to another database on the same network for capacity reasons if there are minor issues. Moving all clients to a secondary data center is not something you always want to do, especially as the secondary data center might not include the same level of bandwidth and may cost more.

HADR

In SQL 11, there is work underway to implement HADR, high availability and disaster recovery, which is an enhanced database mirroring. In this new technology, multiple mirrors are possible, allowing you to have multiple up to date copies of your data in separate locations. These are now called “replicas” and you get up to 4, according to Brent Ozar (HADRON Rocks). I didn’t see that listed, but I’m sure Brent knows more about this stuff than I do.

The other nice things is that you can failover multiple databases together, which you put in an “availability group” and configure together. This is good since many applications might require multiple databases, and it could be important that all databases move to the same server at the same time.

Overall this is cool, and I am looking forward to this technology being deployed. Only one replica works in CTP1, so we can’t really see this working, but it could be an amazing edition to your DR strategy in the future. One note is that this is built on Windows clustering, which no longer requires shared storage, but might require Enterprise Edition, so that will limit how widely this is deployed.

I’ll write more on this later, but the ability to have multiple mirrors, and have multiple databases failover as a group, is something that DBAs should look forward to.