![]() While this automatic failover was occurring, users could not connect to the SQLCLU01A\SQL instance. ![]() As part of the SQL Server startup, any transactions that were in flight and had not committed at the time of the crash were rolled back. The SQLCLU01A\SQL instance started up and connected to all the same databases on the shared storage– there’s one copy of the data, and it doesn’t move. It brought up the SQL Server services on SQLCLU01NODE02. When this happened, the Windows Failover Cluster service saw that it went offline. The SQLCLU01NODE01 server crashed unexpectedly. Oh no! There’s been a failure in our environment! The instance has been configured on port 1433. People connect to the SQL Server instance at SQLCLU01A\SQL. ![]() It has two nodes (servers), which are named SQLCLU01NODE01 and SQLCLU01NODE02. Here is a diagram of a SQL Server cluster. You can even make it always use the same IPAddress and port– so no users of the SQL Server have to know where it is at any given time. It will always have the same instance name, SQL Agent jobs, Linked Servers and Logins wherever you bring it up. Translation: A failover cluster basically gives you the ability to have all the data for a SQL Server instance installed in something like a share that can be accessed from different servers. There is only one copy of the data, but the network name and SQL Server service for the instance can be made active from any cluster node. That allows the cluster to move the SQL instance to any server (or “node”) in the cluster whenever you request, or if one of the nodes is having a problem. Key Concept: A Windows Failover Cluster uses shared storage– typically, this shared storage is on a SAN. When a SQL Server instance is installed on the cluster, system and user databases are required to be on the shared storage. In this post I’m talking specifically about clustering SQL Server 2005 or later using Windows Server 2008 or later. There are lots of types of clusters out there. When we cluster SQL Server, we install one or more SQL Server instances into a Windows Failover Cluster. What Type of SQL Clustering are we Talking About? I’ll also give an overview of how clustering relates to the AlwaysOn Availability Groups feature in SQL Server 2012, and wrap up with frequently asked questions about clustering SQL Server. Today, I’ll tell you what clusters are, what they’re good for, and why I like to plan out my clusters in a very specific way. I was lucky enough to begin working with SQL Server clusters early in my career, but many people have a hard time finding simple information on what a cluster does and the most common gotchas when planning a cluster. The options for high availability can get confusing.
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