1. Root Cause Analysis with DMVs
Using DMVs to gather performance data from SQL Server

2. Intro Introduced with SQL Server 2005
First method for gathering data on SQL Server performance
Advantages:
Allows us to gather information quickly
Data is gathered passively by SQL Server
Disadvantages:
Piecemeal information; does not provide us with a complete data set to review afterward

3. Query Execution Stats Review resource usage by cached plans
sys.dm_exec_query_stats
sys.dm_exec_query_plan
sys.dm_exec_sql_text
Statistics can reach back to last restart

4. sys.dm_exec_query_stats
Contains performance data on cached plans
Logical\physical reads, CPU, executions, duration
2016: adding threads and memory grants
Data relates to statements, not batch
Provides sql_handle and plan_handle
Caveat: Only provides details on cached plans
Restarting SQL Server, flushing the cache empties this

5. sys.dm_exec_query_plan(plan_handle)
Returns the XML plan
Plan_handle provided by
sys.dm_exec_query_stats
Sys.dm_exec_cached_plans
Sys.dm_exec_requests
Same caveats as sys.dm_exec_query_stats
See also sys.dm_exec_text_query_plan

6. sys.dm_exec_sql_text (sql_handle | plan_handle)
Provides the TSQL for a batch
Keeps whitespace and comments
Too long?
Copy\paste to another session

7. Waits and Status Waits are key to understanding performance issues
Review whether queries are waiting, blocked, or active
Check status of the connection
sys.dm_os_wait_stats
sys.dm_exec_requests
sys.dm_exec_sessions
sys.dm_exec_connections
sys.dm_os_waiting_tasks

8. sys.dm_os_wait_stats Key point to understanding SQL Server performance
Running total of all waits
Resets on restart
Can be manually reset:
DBCC SQLPERF ('sys.dm_os_wait_stats', CLEAR);
Need two samples to see changes
Ignore background noise

9. sys.dm_exec_requests
Includes information about each active query\command on SQL Server
Wait types, wait resources, and duration
CPU, reads, and writes
Memory Allocation and Command type
ANSI connection settings and login details
Provides sql_handle, plan_handle, and request_id
Only provides details on parent thread

10. sys.dm_exec_sessions Returns information about all open sessions
Including those not running a query currently
Overlaps some details with sys.dm_exec_requests
Performance details relate to the connection, not current query
Provides client application details (host_name, program_name)

11. sys.dm_exec_connections
1-to-1 relationship with sys.dm_exec_sessions
Excludes system sessions
Connection_id relates to
sys.dm_exec_sessions
sys.dm_exec_requests
Provides client_net_address, most_recent_sql_handle

12. sys.dm_os_waiting_tasks
Provides information on all threads that are waiting or blocking
More useful for parallel queries
sys.dm_exec_requests only provides the parent request
Task_address relates to other OS DMVs
Detailed information on resource_description:

13. Memory Usage Quick approaches for identifying issues
Frequently indicated in Errorlog
Max Server Memory limits buffer pool
Buffer Pool typically largest consumer
Consider other applications on the system

14. sys.dm_os_memory_clerks
Currently active memory clerks
Details not query specific
Use to determine larger trends in memory
Aggregate by type:
MEMORYCLERK_SQLBUFFERPOOL typically largest

15. sys.dm_os_memory_objects
Returns information on all currently allocated memory objects
More granular than sys.dm_os_memory_clerks
Can be grouped and aggregated by type
Multiple counters for various memory allocations
Can relate to other tables by memory_object_address or page_allocator_address

16. sys.dm_exec_cached_plans
One entry for each currently cached plan
Shows the memory allocation for the plan object itself
Provides a usecount and refcount for each and the plan_handle
optimize for ad hoc workloads?
Can breakdown memory objects by type

17. sys.dm_exec_query_memory_grants
Returns details about all queries that have requested memory
Granted and waiting (grant_time IS NULL)
Identify queries waiting on memory and requested amounts
Identify granted requests consuming large amounts of memory
Links by plan_handle, sql_handle, request_id, session_id
Has DOP and Cost information

18. IO Statistics Common performance bottleneck
Expect a read\write to take <=10ms, on average
Need a way to confirm wait types
Source of storage issue:
Overloaded
Underperforming

19. sys.dm_io_virtual_file_stats (database_id, file_id)
Provides details on activity on the specified file
Total reads and writes, IO waits (ms)
Pass NULL parameters to see all files
Calculate delay per read or write
Numbers reset on restart
See relative IO activity within SQL

20. sys.dm_io_pending_io_requests
Provides details on pending IO requests
Requests waiting at the OS or SOS level
Can be related to schedulers (CPU)
Can indicate if IO is impacting a scheduler
io_pending_ms_ticks provides a duration for pending operations
Could indicate storage a bottleneck
Dirty reads are possible; numbers may be inaccurate

21. In summary, Each DMV exposes different parts of SQL Server’s activity
This list is not exhaustive
Data can be gathered and reviewed very quickly
No logs for this data
Unless you create code to write this to a permanent table
Many DMVs are more useful in conjunction

22. Questions
@SQLJared