SQL Server Extended Events

What, Why, How, Who?

Stuart Moore• Started with SQL Server 7 in 1998, 15 years later still working

with it, but newer versions as well.• Worked as DBA and Developer in that period.• Also work with Oracle, MySQL and Linux

• In spare time I’m most likely to be found studying a Mathematics degree with the OU, or sat on a bike saddle somewhere remote.

• Email: stuart.moore@leaf-node.co.uk• Twitter: @napalmgram

What we’ve had

• SQL Trace – Server side tracing mechanism• SQL Profiler – Client side tool to use SQL Trace

What was wrong with that?

• Performance hit– All event data captured, and then filtered– Especially bad if run through Profiler

• Not the most user friendly of syntax:

So, Extended Events

• Introduced in SQL Server 2008.• ‘Extended’ in SQL Server 2008R2 – Unofficial GUI from Codeplex

• And again in SQL Server 2012– Now includes all SQL Trace functionality– Official GUI tool in SSMS

SQL Server 2008 SQL Server 2008R2 SQL Server 2012

action 35 35 48

Event 253 257 616

Map 57 60 240

Pred_compare 111 111 77

Pred_source 29 29 44

Target 7 7 6

Type 29 29 28

Changes across SQL Server versions

• Extendable– New packages can be loaded, for instance for

Microsoft Support calls• Better performance– Filtering done as early as possible to avoid

overheads– You already have a session running and probably

not noticed: • System_health

• Sessions can be told to ‘lose’ events if performance degraded:– Allow_single_event_loss (Default)– Allow_multiple_event_loss– No_event_loss

• Can persist server restarts

Basic Example

• Demo

Packages

• All events, actions, types, etc belong to a Package.

• Registered packages can be seen in– sys.dm_xe_packages

• SQL 2012 ships with 8 packages. Others can be installed, usually by MS support for debugging faults

• Packages loaded by corresponding module during startup.

• All events, objects, targets, etc are owned by a package

• But, all can be used interchangably– Ie; a sqlos event can capture sqlserver actions and

record in a package0 target• Anything marked ‘private’ is system access only:– SecAudit being the prime example

Sessions

• All defined event sessions recorded in– sys.server_event_sessions

• If session is running, it’s recorded in– Sys.dm_xe_sessions

Events• The events which can be monitored.– 616 in SQL Server 2012• Select * from sys.dm_xe_objects where

object_type=‘event’ and isnull(capability,’’)<>’private’

– Each event is ‘owned’ by a package:select b.name, a.* from sys.dm_xe_objects a inner join sys.dm_xe_packages b

on a.package_guid=b.guid where a.object_type='event' and isnull(a.capabilities_desc,'')<>'private'

• A session can capture more than one event:

Create event session ex1 on serveradd event sqlserver.sql_statement_startingadd event sqlserver.sql_statement_completedadd target ring_buffer

‘Payload’

• Each event ‘drops’ a payload to the ‘target’:

select b.name, a.name, a.type_name, a.description, a.column_type, a.column_value From sys.dm_xe_object_columns a join sys.dm_xe_objects b on a.object_package_guid=b.package_guid and a.object_name=b.nameand isnull(b.capability,’’)<>’private’

• 3 column_type values:– readonly – internal value– data – values returned by default– Customizable – these can be changed, options

described in the description field, and default value in the column_value field.

Actions

• Actions are extra data/payload that can be collected when an event fires:

select b.name, b.description, a.* from sys.dm_xe_objects a join sys.dm_xe_packages b on a.package_guid=b.guid where a.object_type='action' and isnull(a.capabilities_desc,'')<>'private';

Predicates

• Used to filter the events captured:select b.name, a.* From sys.dm_xe_objects a join sys.dm_xe_packages b on a.package_guid=b.guid where a.object_type='pred_source' and isnull(a.capabilities_desc,'')<>'private';

Maps

• Means of mapping names to values for predicates– For example Wait types to an ID

select b.name, a.name, a.map_key, a.map_value, b.description from sys.dm_xe_map_values ainner join sys.dm_xe_objects b on a.object_package_guid=b.package_guid and a.name=b.nameorder by b.name, a.map_key

Targets

• Where the data ends up.– A number of different types, main differences:• Synchronous• Asynchronous• Memory resident• Persisted storage (disk)

• Etw_classic_sync_target– Used for ETW, useful for devs for tracing through large

systems but out of scope here• Histogram & event_counter– Memory resident tally targets. Histogram used to group

data asynchronosyly, counter is a synchronous counter• Pair_matching– Memory resident and asynchronous. Used to pair up

events, eg; beginning and end of transaction• Event_file– Disk based asynchronous target, for bulk or long term

retention• Ring_Buffer– Memory based asynchronous FIFO target.

Examples

• 1 – Deadlocks• 2 – Possible parameter sniffing• 3 – Capture Data file growth• 4 – Statement counting• 5 – Perfmon stats via GUI

All Good?

• Not quite:– Can still drag performance down,– Viewing results in GUI still a resource hog

• But it’s the way forward

References

• Jonathan Kehayias:– on the load impact of Extended Event sessions:

• http://bit.ly/XLeMWF

– 31 days of Xevents:• http://bit.ly/153GfZU

• MSDN– Overview

• http://bit.ly/13eCnCx

– Dynamic Management View• http://bit.ly/WWg4T1