Oracle - SQL - MySQL DBA - Life Long Learning Experience

In this article we'll look at how AlwaysOn availability groups combine with Failover Cluster Instances. We'll look individually at Failover Cluster Instance composition, Windows Server Failover Clusters and AlwaysOn availability groups, what they are and how they interact. For the purposes of this article the following are used; FCI     A failover cluster instance of SQL Server AO     Always On availability groups, the new SQL Server 2012 feature WSFC     Windows Server Failover Cluster, a feature of the Windows 2008 operating system NIC     Network interface card SAN     Storage area network LUN     Logical unit number  For many years now Windows Server Failover Clusters and Failover Cluster Instances have been a popular configuration for highly available SQL Servers. Later, with SQL Server 2005 came database mirroring. Although mirroring was limited, it offered an extra level of redundancy. Now with SQL Server 2012, we take a huge leap forward in highly available SQL Servers.

When a transaction is being committed, each step is written into the transaction log. Each value that is being inserted, updated, or deleted is recorded so that if there is a failure before the log is written to the data file, the appropriate rollback or roll forward can take place. The transaction may take and release locks on tables or indexes. To view the entries in the log there is an undocumented fn_dblog command . The SQL Server transaction log contains the history of every action that modified anything in the database. Reading the log is often the last resort when investigating how certain changes occurred. It is one of the main forensic tools at your disposal when trying to identify the author of an unwanted change. Understanding the log and digging through it for information is pretty hard core and definitely not for the faint of heart. And the fact that the output of ::fn_dblog can easily go into millions of rows does not help either. But I’ll try to giv