Lesson 5: Correlating Performance and Monitoring Data in Visual Studio .NET

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Lesson 5: Correlating Performance and Monitoring Data
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SQL Server uses a cooperative multiprocessing model instead of a symmetric multiprocessing model for processing queries. Controlling this cooperative query processing behavior is the job of the UMS. By correlating all the information at your disposal from SQL Server Profiler, System Monitor counter logs, and operational statistics in DMVs/DMFs, you can target the root cause of a performance issue. To correlate this information, your SQL Server Profiler trace must capture Start Time data.
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The following questions are intended to reinforce key information presented in this lesson. The questions are also available on the companion CD if you prefer to review them in electronic form.
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Answers to these questions and explanations of why each answer choice is right or wrong are located in the Answers section at the end of the book.
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1. Which data column is required to correlate a System Monitor counter log to a trace in SQL Server Profiler A. Text Data B. End Time C. SPID D. Start Time
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Monitoring and Troubleshooting SQL Server Performance
Lesson 6: Resolving Blocking and Deadlocking Issues
If all databases were read-only, we wouldn t have to deal with concurrent access issues. However, we also wouldn t have to worry about any data. Any database that allows multiuser access and data modifications must have mechanisms to maintain data consistency. Having locking and blocking is a desired effect. However, having locking or blocking for an extended period of time or having deadlocks is undesirable and must be resolved. This lesson discusses the locking mechanisms that SQL Server uses to manage concurrent access, how to minimize the effect of blocking, and how to avoid deadlocks.
After this lesson, you will be able to:
Identify causes of a block by using DMVs. Terminate processes. Configure SQL Server Profiler for deadlock events. Log deadlock chains to the SQL Server error log. Analyze deadlock chains. Understand how isolation levels affect blocking. Understand how transactions can cause blocking in multiuser systems.
Estimated lesson time: 45 minutes
Understanding Locking
To manage multiuser access to data while maintaining data consistency, SQL Server uses a locking mechanism for data. This mechanism arbitrates when a process is allowed to modify data as well as ensuring that reads are consistent. Locks occur at three different levels and can be of three different types. A lock can be applied at a row, page, or table level. SQL Server manages the resources allocated by locks and determines the appropriate level of the lock based on a relatively aggressive escalation mechanism.
NOTE
Do database-level locks exist
You might find a database-level lock mentioned in some texts about SQL Server. This type of lock does not exist. Some people use this term simply to indicate that SQL Server has acquired a tablelevel lock on all tables within a database.
Lesson 6: Resolving Blocking and Deadlocking Issues
The main decision threshold occurs at approximately three percent to five percent. If SQL Server determines that a query requires locks on three percent to five percent of the rows on a given page, it acquires a page-level lock. Similarly, if SQL Server determines that a query requires locks on three percent to five percent of the pages in a given table, it acquires a table-level lock. Because it is not always possible to accurately predict the percentage of rows or pages that require a lock, SQL Server can automatically promote from fine-grained locks to a coarser level of lock. This process is called lock escalation.
NOTE
Lock escalation paths
It is a common misconception that SQL Server escalates locks from a row level to a page level and finally to a table level. However, lock escalation has exactly two paths. SQL Server escalates rowlevel locks to table-level locks, and it escalates page-level locks to table-level locks.
In addition to the locking levels, SQL Server has three types of locks: shared, exclusive, and update. A shared lock, as its name implies, allows shared access to the data. An unlimited number of connections are allowed to read the data. However, any piece of data that has a shared lock on it cannot be modified until all shared locks are released. An exclusive lock, as its name implies, allows only a single connection to access the locked data. SQL Server uses this type of lock during data modification to ensure that other users cannot view the data until it has been committed to the database. An update lock is a special case. This lock begins as a shared lock while SQL Server locates the rows it must modify within the table. After SQL Server locates the rows, it promotes the lock to an exclusive lock just before it performs the actual modification of the data. This lock promotion during an update is the most common cause of deadlock issues, which we will cover in a moment.
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