The Importance of Wait Statistics in SQL Server
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Wait statistics provide important information for monitoring and improving SQL Server performance. They indicate where the server is spending time waiting on resources and which resources are causing frequent or long waits. Key wait statistics are available in dynamic management views like sys.dm_os_wait_stats and sys.dm_db_wait_stats. Some waits like PAGEIOLATCH_IO_COMPLETION are more important to focus on than others like CXPACKET. Wait statistics can help identify issues related to the system, queries, or database structure. Extended events and other views also provide related wait information down to the query level.
The Importance of Wait Statistics in SQL Server
- 1. The Importance of Wait Statistics in Monitoring SQL Server Grant Fritchey
- 3. What is a Wait? • SQL Server does work • All the work doesn’t occur simultaneously • When a unit of work stops • Threads – RUNNING – SUSPENDED – RUNNABLE • Time spent between RUNNING and RUNNING
- 4. Why Are Waits Important? • Why is the server slow • Where is the server slow • Which resources are waiting frequently • Which resources are waiting long • Direct pointer to the cause of poor performance
- 5. Where Do I Get Waits • Sys.dm_os_wait_stats • Sys.dm_db_wait_stats • sys.dm_os_latch_stats
- 6. Are Some Waits More Important? • LATCH_*, PAGELATCH_* • PAGEIOLATCH_*, IO_COMPLETION, WRITELOG • CXPACKET • ASYNC_NETWORK_IO • RESOURCE_SEMAPHORE • BACKUPIO • Lots and lots more • signal_wait_time_ms
- 7. Are Some Waits Less Important? • CXPACKET • WAITFOR • CLR_SEMAPHORE • BROKER_RECEIVE_WAITFOR • ONDEMAND_TASK_QUEUE • Lots and lots more
- 8. How Do I Use This Information? • Filter out the unimportant waits • You know where your system bottlenecks are • Is it system related? • Is it query related? • Is it structure related?
- 9. How Do I Know Which Queries Are Waiting? • Sys.dm_os_waiting_tasks
- 10. How Do I Know Which Waits A Query Has? • Sys.dm_exec_requests • Extended Events: wait_info
- 11. What Happens To Waits Over Time? • DBCC SQLPERF(“sys.dm_os_wait_stats”,CLEAR) • DBCC SQLPERF(“sys.dm_os_latch_stats”,CLEAR) • Instance restart
- 12. DEMO: Wait Stats in SQL Monitor
- 13. SQL Monitor • Clear insights into SQL Server performance in real time • Embedded expertise • Alerting • Baselines • Wait stats & expensive queries • Custom Metrics • SQL Monitor
- 14. SQL Monitor • Read more experiences of SQL Monitor from real customers at: www.red- gate.com/products/dba/sql-monitor • We use it to monitor SQLServerCentral, if you want to see how it works: http://monitor.red-gate.com
- 15. Its very simple to use and doesn’t drown me in unnecessary information. It came out on top for being reasonably priced and also functionality. We now have a stable SQL Estate due to SQL Monitor. Nic Hopper, Senior DBA, Atrium Underwriters Ltd
- 16. Documents • SQL Server Performance Tuning Using Wait Statistics • Performance Tuning Using Waits and Queues (old, but still good) • Wait statistics, or please tell me where it hurts – Paul Randal • What Queries Are Causing My Waits – John Sterret
- 17. Thank you for your attention Q & A