Editorials

The Cost of Failover

by Ben Taylor

The Cost of Failover
We were talking about two mainstream High Availability techniques you can consider to keep your database system online. Today we want to talk about the cost of High Availability in order to help you decide if HA is the correct solution for your implementation.

Rather than providing exact costs, I think it will be helpful to list out the resources you will require in order to use High Availability solutions. Then you can extrapolate from that list what your specific implementation may be.

SQL Server clustering

  • SAN with high speed connection to server
  • Two or more servers
  • Windows and SQL Server license. If you wish to use both servers as the primary host for different databases, then you must purchase licenses for both servers. If one server is solely for failover, you may purchase only one SQL Server License.

Mirroring

  • Two or more servers.
  • Each server must be fully licensed for Windows and SQL Server
  • Each Server must have it’s own disk storage. It can use a SAN, but nothing is shared between instances. It is entirely duplicated.
  • A witness server (recommended as separate installation from the other two servers). The witness server will require a Windows OS, but may run the Express version of SQL Server.

Replication

  • Replication is essentially the same as mirroring. It does not require a witness server.

In all scenarios it is recommended the capabilities of all servers be equivalent. Identical servers used to be required for clustering, and is still considered a good practice.

Given these costs for duplicate hardware and software, you can see that High Availability has a significant cost associated with it.

Is HA the correct solution for you? If you can’t afford it, what is the cost of outage? How long will it take you to get your system back online? We’ll talk about that more tomorrow.

Please write to btaylor@sswug.org if you would like to share your experience in this area.

Cheers,

Ben

$$SWYNK$$

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Author: Ben TaylorBen is a Relational Database veteran starting in the early eighties on DEC Vax database systems such as Bloom and Harvest. He has specialized in SQL engines for more than two decades, with a primary focus on Microsoft SQL Server. Ben has worked with small and large databases for both OLAP and OLTP working on systems for many industries such as Medical (Medical Claims and Eligibility Warehouse, HMO Provider Management, Enrollment), Financial (credit card processing, mortgage, stock portfolio management), Manufacturing (Assembly Line and Product Assemblies), Sales, Auto Auctions and more.