Troubleshooting sql server

C H A P T E R

Troubleshooting SQL Server

Athens Oct 23, 2015

Presenter introduction

1982 - I have been started with computers.

1988 - I started my professional carrier in computers industry.

1996 - I have been started to work with SQL Server version 6.0

1998 - I earned my first certification at Microsoft as Microsoft CertifiedSolution Developer (3rd in Greece) and started my carrier as Microsoft Certified Trainer (MCT) with more than 25.000 hours of training until now!

2010 - I became for first time Microsoft MVP on SQL ServerI created the SQL School Greece (www.sqlschool.gr)

2012 - I became MCT Regional Lead by Microsoft Learning Program.

2013 - I was certified as MCSE : Data Platform, MCSE : Business Intelligence

Antonios ChatzipavlisDatabase A rchitect

SQL Se rve r Evangel ist

MCT, MCSE, MCITP, MCPD, MCSD, MCDBA,

MCSA, MCTS, MCAD, MCP, OCA, ITIL-F

Follow us in social media

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Presentation outline

01: SQL Server Monitoring Overview

02: SQL Server Threading Model

03: Using Wait Statistics

04: Using Extended Events

05: Create your Baseline

C H A P T E R

01SQL Server Monitoring Overview

Why Monitor?

Guidelines for Monitoring

Monitoring Tools for SQL Server

Why Monitor?

Diagnosing causes of performance issues Response Time

Throughput of the system

Detecting and resolving concurrency issues SQL Server uses locks to ensure data consistency.

Identifying changing trends in resource utilization Capacity planning to identify the required hardware resources to support the

database workload.

It is important to monitor resource utilization changes

Guidelines for Monitoring

Understand the workloads you need to support Every workload has different requirements

You need to understand its workloads so that you can identify the relevant metrics to monitor

Prioritize resources based on the importance of each workload to the business

Establish a baseline A common mistake is to wait until there is a problem before monitoring the SQL Server solution

A better approach is to identify the key metrics that your workloads rely on, and record baseline values for these metrics when the system is operating normally

Monitor regularly to track changes in key metrics. It is generally better to proactively monitor the system on a regular basis to identify any trends

that signify changes in the way the workloads consume resources.

With this approach, you can plan server upgrades or application optimizations before they become critical.

Monitoring Tools for SQL ServerBuild in and External Tools

Activity MonitorA component of SQL Server Management Studio that

enables DBAs to view details of current activity in the

database engine

Dynamic Management

View and FunctionsDatabase objects that provide insight into internal SQL Server operations

Performance MonitorA windows administrative tool that you can use to record values for multiple performance counters over a period of time, and analyze results in a variety of chart

SQL Server ProfilerA tracing and profiling tool that you can use to record

details of Transact-SQL and other events in a SQL Server

workload. We can replay or use it as a source for tuning

SQL Trace A lightweight, Transact-SQL based programming interface

for tracing SQL Server activity


Database Engine Tuning Advisor

A tool provided with SQL Server for tuning indexes and statistics based on a known workload

SQL Server Utility Control Point

A centralized management portal for monitoring server health for multiple instances based on specific collection sets

SQL Server Extended Events

A lightweight eventing architecture.

SQL Server Data

Collection

An automated system for collecting and storing and

reporting performance data for multiple SQL Server

instances

Distributed Replay An advanced tool for replaying workloads across a potentially distributed set of servers


Microsoft System Center

Operations Manager

An enterprise-wide infrastructure management solution that uses management packs to collect performance and health data from Windows and application services

SqlDiag ToolThe SQLdiag utility is a general purpose diagnostics collection utility that can be run as a console application or as a service

sp_WhoIsActiveProvides detailed information about all of the sessions running on your SQL Server system, including what they’re doing and how they’re impacting server behavior

sp_Blitz SQL Server health and warning check

Performance MonitorWindows Performance Monitor is an operating system tool that brings together several previously disparate performance and monitoring tool

Monitoring Tools for SQL ServerSQL Server 2016

Query StoreAn enterprise-wide infrastructure management solution that uses management packs to collect performance and health data from Windows and application services

Live Query StatsWindows Performance Monitor is an operating system tool that brings together several previously disparate performance and monitoring tool

C H A P T E R

02SQL Server Threading Model

Introduction

What a thread is

Thread Scheduling

SQL Server Schedulers

Thread States

The Waiter List

The Runnable Queue

Introduction

SQL Server is an OS on Windows OS

It performs and controls its own: Memory management

I/O

Scheduling

What a thread is

A thread is unit of execution within a process

Multiple threads can exist within a process Each is given small amount of processor time

Waits while other threads execute (called scheduling)

SQL Server uses OS threads to perform its work Called worker threads

Dedicated threads exists

Such as for performing checkpoints, deadlock monitoring, and ghost record cleanup

Most are in a pool of threads that SQL Server uses to process user requests

(Working Threads)

Thread Scheduling

SQL Server has its own thread scheduling Called non-preemptive scheduling

Is more efficient than Windows scheduling

Performed by the SQLOS layer of the Storage Engine

Each processor core (logical or physical) has a scheduler A scheduler is responsible for managing the execution of work by threads

Schedulers exist for user threads and for internal operations

Use the sys.dm_os_schedulers DMV to view schedulers

SQL Server Schedulers

ProcessorRunnable Queue

Waiter List

One scheduler per logical or physical processor core

Plus some extra ones for internal tasks and the DAC

Thread States

RUNNING The thread is currently executing on the processor

SUSPENDED The thread is currently on the Waiter List waiting for a resource

RUNNABLE The thread is currently on the Runnable Queue waiting to execute on the

processor

The Waiter List

Is an unordered list of threads that are suspended Any thread can be notified at any time that the resource it is waiting for is now

available

No time limit for a thread on the waiter list Although execution timeouts or lock timeouts may take effect

No limit on the number of threads on waiter list

The sys.dm_os_waiting_tasks DMV Shows which threads are currently waiting and what they are waiting for

The Runnable Queue

Is a strict First-In-First-Out (FIFO) queue Resource Governor exception

The sys.dm_os_schedulers DMV shows the size of the

Runnable Queue in Runnable_tasks_count column

Examine Schedulers

02

Examine Schedulers

Examine Waiting Tasks

C H A P T E R

03Using Wait Statistics

Waits and Queues

Wait Times

DMVs for Waits

Waits and Queues

Waits Wait is when a thread that's running on the processor cannot proceed because

it needs a resource and the resource that it needs is not available

The thread state changed from RUNNING to SUSPENDED

Moved to Waiter List and remains on the list until it became available

Queues Is a generic term and means what's stopping the thread being able to get its

resource

PAGEIOLATCH_XX : The I/O subsystem need to complete the I/O

LCK_M_XX : Another thread holding an incompatible lock

CXPACKET : Another thread needs to complete its portion of work

Wait TimesWait time=Resource Wait time + Signal Wait time

Wait time Is the time spent from when a thread is running on the processor (RUNNING)

moved to the Waiter List (SUSPENDED) and is being signaled and moving back

onto the runnable queue (RUNNABLE) and is going back to running

Resource Wait time Time spent on the Waiter List with state SUSPENDED

Signal Wait time Time spent on the Runnable Queue with state RUNNABLE

DMVs for Waits

sys.dm_os_waiting_tasks Shows all threads that are currently suspended

Think of it as the ‘what is happening right now?’ view of a server

The first thing to run when approaching a ‘slow’ server

sys.dm_os_wait_stats Shows aggregated wait statistics for all wait types

Aggregated since the server started or the wait statistics were cleared

Some math is required to make the results useful

Calculating the resource wait time

Calculating the average times rather than the total times

Using Waits DMVs

03

C H A P T E R

04Using Extended Events

Why Extended Events?

Switch from SQL Trace to Xevents

Extended Events Architecture

Event Life Cycle

Targets

Why Extended Events?

It’s the FUTURE SQL Trace is a deprecated feature in SQL Server 2012

This makes understanding XE crucial to supporting SQL Server in the future

Less overhead Lightweight to minimize impact

Provides minimum schema of data that is specific to the event being fired

Events are filtered early in the firing lifecycle based on the predicates

Flexibility and Power Allows complex configurations for event collection that simplify problem identification.

Many events in more recent releases

Switch from SQL Trace to XEvents

Events Mapping Query

Column to Action Mapping Query

select xe.xe_event_name,st.namefrom sys.trace_xe_event_map as xeinner join sys.trace_events as ston xe.trace_event_id = st.trace_event_id;

select xe.xe_action_name, tc.namefrom sys.trace_xe_action_map as xeinner join sys.trace_columns as tcon xe.trace_column_id = tc.trace_column_id;

Extended Events Architecture

Sessions Are a functional boundary for configuration of events

Events Correspond to well-know points of code

Predicates Boolean expressions that define the conditions required for an event to actually fire

Actions Actions only execute after predicate evaluation determines the event will fire

Targets Targets are event consumers

Event Life Cycle

Event point

encountered in code

Is Event Enabled

in a session Code Continues

Buffer Data for

Asynchronus Targets

Send to Synchronous

Targets Immediately

Execute Actions and

Collect data

(if applicable)

Are there

configurable

columns

Collect

non-configurable

column data

Passes Filter

Criteria (Predicate)

Collect Configurable

Column data

No

No

Yes

Yes

Yes

Targets

Event counter Counts all specified events that occur during an Extended Events session.

Use to obtain information about workload characteristics without adding the

overhead of full event collection.

This is a synchronous target.

Event file Use to write event session output from complete memory buffers to disk.

This is an asynchronous target.

Targets

Event pairing Many kinds of events occur in pairs, such as lock acquires and lock releases.

Use to determine when a specified paired event does not occur in a matched

set.


Event Tracing for Windows (ETW) Use to correlate SQL Server events with Windows operating system or

application event data.

This is a synchronous target.

Targets

Histogram Use to count the number of times that a specified event occurs, based on a

specified event column or action.


Ring buffer Use to hold the event data in memory on a first-in first-out (FIFO) basis, or on a

per-event FIFO basis.


Use case scenarios of Extended Events

04

C H A P T E R

05Create your Baseline

Create Baseline

05

Summary

SQL Server Monitoring Overview

SQL Server Threading Model

Using Wait Statistics

Using Extended Events

Create your Baseline

Questions?

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Troubleshooting sql server