SQL Server Monitoring and Performance

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Chapter 9: Monitoring and Performance

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Course 157: SQL Server 2008 Database Administration

Presented by Scott Whigham

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w • Chapter Introduction

• Tools for Monitoring

• SQL Server Profiler M

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g • Understanding and Using the DMVs

• PerfMon

• Data Collector

What We’re Going to Cover

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• Chapter 9 is about performance and exception monitoring your server

• We’ll start with the basics (using Profiler) and end with new features in SQL Server 2008 (Data Collector)

Chapter Introduction

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• PerfMon

• Data Collector


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• There are many built-in tools to monitor performance and health:– Activity Monitor and sp_who

• Covered in Chapter 5!

– SQL Server Profiler• Detailed view of statements issued against instance

– Dynamic Management Views (DMVs)• “Live” view of server health and activity

– Performance counters and objects (“PerfMon”)• “Live” view of server health and activity

– Data Collector• Historical view of server health and activity

Tools of Performance Monitoring

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Profiler DMVsPerfMon

Data Coll.

Provides “live”, real-time reporting?

Can do trend analysis and historical reporting? Requires manual load

Requires manual load


Can view SQL issued?

Provides detail-level view or summary? Detail Depends… Summary Summary

Can be used to diagnose performance problems?

Level of impact on server Low None Negligible Low-Hi

Difficulty level 100-300 200-400 200-300 300-500


“But Mr. Scott, how do I know when to use each one?”

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• Let’s play with Profiler!

In the next video…

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• PerfMon

• Data Collector


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• SQL Server Profiler is a tool for monitoring real-time queries issued against your server– “Old Faithful” - been around for years

– Requires sysadmin or ALTER TRACE server permission

SQL Server Profiler

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SQL Server Profiler

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• DBAs use traces to monitor statements sent to SQL Server– We’ve already seen how SQL Server uses “default

traces” in Chapter 7

• Running traces will have an impact on server performance– How much? It depends on how much you trace

SQL Server Profiler

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• SQL Server Profiler is the GUI for defining, viewing, and replaying traces– It does not have to be running to create a trace

– Many DBAs create their traces via Profiler and then script out to jobs

SQL Server Profiler

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• Running Profiler on a “live” system adds to an already stressful situation– Particularly if run interactively on the server

• If you must run Profiler against a “live” server, run it remotely

• Adds network overhead but this is almost never an issue today

SQL Server Profiler

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• Trace data can be stored:– In a .trc file (fastest)

– In a SQL Server table

• .trc files can be configured to rollover to a new file after a certain threshold

• Can load .trc file contents into SQL Server table after trace for maximum flexibility!

SQL Server Profiler

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• After trace is complete, you can export results

SQL Server Profiler

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• Tips on working with Profiler– Traces can have filters to help minimize the noise

• The better you filter, the less of an impact you put on the server

– Being able to write queries against the trace results can help you identify many common trends and problems

• This requires you load the trace data into a table!

SQL Server Profiler

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• Let’s play with Profiler!


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• PerfMon

• Data Collector


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• DBAs use Dynamic Management Views (“DMVs”) and Dynamic Management Functions (“DMFs”) to peek under the covers– In the sys schema and begin with dm_

– Used to get up-to-the-second performance information during critical time

– Often need to JOIN many together to come up with desired results

Dynamic Management Views

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Profiler DMVsPerfMon

Data Coll.

Provides “live”, real-time reporting?

Can do trend analysis and historical reporting? Requires manual load



Can view SQL issued?

Provides detail-level view or summary? Detail Depends… Summary Summary

Can be used to diagnose performance problems?

Level of impact on server Low None Negligible Low-Hi

Difficulty level 100-300 300-500 200-300 300-500


Reminder…

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• There are too many DMxs to cover each one so we’ll cover concepts and core DMxs– Over 130 DMVs and DMFs in SQL Server 2008!

• It is important for DBAs to maintain a script library


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• The easiest way to get to know the DMVs/DMFs is to categorize them– Below are the more popular categories for

performance monitoring


Category DMx prefix

Database sys.dm_db_

Execution/Sessions sys.dm_exec_

IO sys.dm_io_

OS sys.dm_os_

Transactions sys.dm_tran_

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• Intellisense can help you greatly!


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• Object Explorer can also help


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• You can also query system views for full list

SELECT name, type_desc

FROM sys.system_objects

WHERE schema_id=4

AND name LIKE 'dm_%‘

ORDER BY name

DMV and DMF Query

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DMV and DMF Query Results

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• Database category: – 16+ DMVs and DMFs



WHERE schema_id=4

AND name LIKE 'dm_db%'

Database Category

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• We’ll take a look at a few categories and examples


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• PerfMon

• Data Collector


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• Reminder: the more popular categories for performance monitoring


Category DMx prefix

Database sys.dm_db_


IO sys.dm_io_

OS sys.dm_os_


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• Database category: – 16+ DMVs and DMFs



WHERE schema_id=4

AND name LIKE 'dm_db%‘

ORDER BY name

Database Category

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• From a performance standpoint, you will spend a lot of time in the Database category– Primarily looking at table, index, and partition

information

• Size, fragmentation

• Missing indexes

• Index use

Database Category

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• Common index-related DMVs in the Database Category– sys.dm_db_missing_index_details

returns information about indexes that could potentially help speed up the system

– sys.dm_db_index_usage_stats can tell you whether an index is being used and, if so, in what way

Database Category

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• Common index-related DMFs in the Database Category– sys.dm_db_index_physical_stats is

one way to identify fragmentation (see Chapter 8!)

– sys.dm_db_index_operational_stats

is similar to the DMV sys.dm_db_index_usage_statsyet only includes actual execution events instead of plan generation events

Database Category

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• We’ll take a look at writing queries using the DMVs and DMFs in the Database category


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• PerfMon

• Data Collector


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• Let’s look at the other categories now…


Category DMx prefix

Database sys.dm_db_


IO sys.dm_io_

OS sys.dm_os_


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• Execution/Session category: – 20+ DMVs and DMFs



WHERE schema_id=4

AND name LIKE 'dm_exec%'

Execution/Session Category

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• The Execution/Session category will help you spy on people– That’s what you really want to do, isn’t it?

– Can also view specific query information

• Plans, text, stats, caching information


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• There is at least one DMF in this category that you definitely want to know

–sys.dm_exec_sql_text allows you to

see the query text a user submits

• By itself, it is not that useful but combined with other Execution/Session DMVs it is very powerful

• Accepts one parameter - the memory location of the query plan -

– Populated via a column found in other sys.dm_exec_DMVs


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• A common DMV in this category is:– sys.dm_exec_query_stats allows you to see

metrics about queries submitted to server

• Reads, writes, waits, execution count, etc

• plan_handle column is the column to pass to sys.dm_exec_sql_text to return SQL


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SELECT TOP(50) sql.text

, qs.last_execution_time

, qs.execution_count

, qs.last_elapsed_/1000000.0 as

ElapsedTimeInSecondstime

FROM sys.dm_exec_query_stats qs

CROSS APPLY

sys.dm_exec_sql_text(qs.sql_handle) AS

sql

ORDER BY qs.execution_count DESC

Return 50 most-run queries

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Return 50 most-run queries

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• Other common DMVs in this category are– sys.dm_exec_procedure_stats same as

query_stats yet only returns stored procedure info

– sys.dm_exec_cached_plans gives you info on the cached query plans

• Is it compiled or adhoc?

• How many times has it been used?


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• Spying on users is easy!– sys.dm_exec_sessions is one row for each

authenticated session

-- Returns all non-internal sessions

SELECT * FROM sys.dm_exec_sessions

WHERE session_id>50


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• We’ll write several queries and see the Execution/Session category in action


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• PerfMon

• Data Collector


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• Let’s look at the IO and OS categories now to help diagnose disk pressure


Category DMx prefix

Database sys.dm_db_


IO sys.dm_io_

OS sys.dm_os_


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• IO category: – 4+ DMVs and DMFs



WHERE schema_id=4

AND name LIKE 'dm_io_%‘

ORDER BY name

IO Category

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• The most common item in this category is– sys.dm_io_virtual_file_stats()

returns information about usage of the database files

• Reads, writes, IO waits, size

• Accepts two parameters:– database_id - use NULL for all databases

– file_id - the file_id from sys.database_files; use NULL for all files

IO Category

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• sys.dm_io_virtual_file_stats

returns several columns:

IO Category

Column Description

sample_ms Milliseconds since server was restarted

num_of_reads Physical reads of objects in database file

num_of_bytes_read Large textual-based columns will affect this greatly

io_stall_read_ms Total time a user process waited for IO to this file

num_of_writes, num_of_bytes_written, io_stall_write_ms

Database write activity

io_stall Total amount of waits reading /writing to this file

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• Here are some things to monitor with dm_io_virtual_file_stats()

– Look at total reads+writes to find “most active”

– Look at io_stall to identify files that might benefit from new disks

• All databases have stalls; the important metric is “What percentage of the uptime of the server am I having stalls?”

• Compare read/write stalls on log and data file to identify which could benefit from new/faster disks

IO Category

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-- View IO stats for all DBs on instance:

SELECT DB_NAME(mf.database_id) AS DB

, mf.physical_name, mf.type_desc

, vfs.*

FROM

sys.dm_io_virtual_file_stats(null,null)vfs

JOIN sys.master_files mf

ON mf.database_id = vfs.database_id

AND mf.file_id = vfs.file_id

ORDER BY DB_NAME(mf.database_id)

IO DMF

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IO DMF

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-- Aggregate total IO since last restart:

SELECT

DB_NAME(vfs.database_id) AS database_name,

CAST(SUM(vfs.num_of_bytes_read +

vfs.num_of_bytes_written) / 1048576.

AS DECIMAL(12, 2)) AS IO_MB

FROM sys.dm_io_virtual_file_stats(NULL, NULL) vfs

GROUP BY vfs.database_id

ORDER BY IO_MB DESC

IO DMF

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• The OS category is one of the largest– 36+ DMVs (no DMFs!)



WHERE schema_id=4

AND name LIKE 'dm_os_%‘

ORDER BY name

OS Category

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• You can use this category to find waiting tasks:– sys.dm_os_wait_stats - aggregated info on

dozens and dozens of wait types

– sys.dm_os_waiting_tasks - detail info on waits for a specific resource

• Hint: use dm_os_wait_stats to identify the areas of contention and then dm_os_waiting_tasks to view task information

OS Category

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• These two DMVs are accumulative– Their statistics are the totals since SQL Server

service has been started

• To track effectively over time, it is necessary to reset periodically– Common to store in a table for historical

perspective– DBCC SQLPERF(‘<dmv_name>’, CLEAR)

OS Category

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-- Current wait info:

SELECT wait_type, wait_time_ms / 1000. as wait_time_s

FROM sys.dm_os_wait_stats

ORDER BY wait_time_ms DESC

-- Clear out stats:

DBCC SQLPERF ('sys.dm_os_wait_stats', CLEAR)

-- Run this at a later time to view waits since CLEAR

SELECT wait_type, wait_time_ms / 1000. as wait_time_s

FROM sys.dm_os_wait_stats

ORDER BY wait_time_ms DESC

OS Category

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OS Category

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• To identify disk pressure, you can look in sys.dm_os_wait_stats for specific wait types– Order by wait_time_ms DESC to identify most often

waits– If anyone of these are in the top two positions, you

have disk I/O pressure:• ASYNCH_IO_COMPLETION • IO_COMPLETION • LOGMGR • WRITELOG • PAGEIOLATCH_x

OS Category

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• Let’s look further at the OS category and others


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• PerfMon

• Data Collector


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• The OS category has lots of info– sys.dm_os_sys_info - info such as number

of CPUs, start time of service

– sys.dm_os_tasks - lists each running task

– sys.dm_os_loaded_modules - lists each loaded DLL that SQL Server is using

• Cryptography Manager

• Kerberos Security Package

• Microsoft SQL Server Native Client 10.0

• SQL Server Windows 95 Lite SCM

OS Category

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• Many OS category DMVs can help diagnose memory pressure– Performance Counters can help too

– We’ll cover Perf Counters in an upcoming section of this chapter

OS Category

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• The main areas for memory pressure are:– Database page cache (used to cache table/index)

– Query Workspace memory (for Hash and Sort ops)

– Plan cache

– There are others…

OS Category

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-- Find out what has the most entries

-- in memory

SELECT TOP (20)[Name], COUNT(*) AS 'Count'

FROM sys.dm_os_memory_cache_entries

GROUP BY [Name]

ORDER BY COUNT(*) DESC;

OS Category

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OS Category

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-- Top 10 consumers of Buffer Pool

SELECT TOP 10 [type]

, SUM(single_pages_kb) AS memory_in_kb

FROM sys.dm_os_memory_clerks

GROUP BY type

ORDER BY SUM(single_pages_kb) DESC;

OS Category

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OS Category

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• One area that many servers have trouble with is the TokenAndPermUserStore cache– An internal security system that generates security

tokens for sessions

– Particularly troublesome area for DBs with lots of ad-hoc SQL

– The problem can be that the TokenAndPermUserStore cache takes up so much memory and is so frequently accessed that when there are a high number of items in the cache, access takes a long time

OS Category

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1. Queries for diagnosing TokenAndPermUserStore cache issues:

-- What is the size? The larger, the slower!

SELECT SUM(single_pages_kb + multi_pages_kb)

AS ‘CurrentSizeOfTokenCache(kb)’

FROM sys.dm_os_memory_clerks

WHERE name = 'TokenAndPermUserStore'

OS Category

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2. Queries for diagnosing TokenAndPermUserStore cache issues:

-- How many entries in the

-- TokenAndPermUserStore?

SELECT TOP (20)[Name], COUNT(*) AS 'Count'

FROM sys.dm_os_memory_cache_entries

GROUP BY [Name]

ORDER BY COUNT(*) DESC;

OS Category

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Scary!

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• There are multiple ways to prevent TokenAndPermUserStore issues1. Minimize ad-hoc SQL

2. Log in as a sysadmin (no tokens need to be generated)

3. Limit the size of the cache via sp_configureusing• access check bucket count

• access check cache quota

• http://support.microsoft.com/kb/955644

OS Category

http://support.microsoft.com/kb/955644

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• There is one way to fix a “live” TokenAndPermUserStore issue– Free the system cache

-- WARNING: Do not use on production without

-- a full understanding!

-- After running, there will not be an entry

-- in sys.dm_os_memory_clerks for TAPUS!

DBCC FREESYSTEMCACHE('TokenAndPermUserStore')

OS Category

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• Let’s look at blocking and locking…


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• To maximize coverage, you want to use both DMVs/DMFs along with Performance Monitor– a.k.a. PerfMon and System Monitor

• PerfMon gathers stats about SQL Server as well as the rest of the environment– Disk subsystems, non-SQL resource usage

Performance Monitor

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• PerfMon uses objects and counters– Objects are for categorization

– Objects contain one or more counters

• When you install SQL Server, it installs a set of instance-specific objects

Performance Monitor

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• You can view the SQL Server objects and counters using the DMV sys.dm_os_performance_counters

– Careful - more than 1,100 rows

Performance Monitor

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• A few things to know about sys.dm_os_performance_counters

– Only displays SQL Server instance’s counters

– Any counter that has “/sec” in the name is cumulative since time SQL Server service was started• Transactions/sec

• Bulk copy rows/sec

– To accurately read these values, compare intervals• Compare 0800 reading to 0900 reading, for

example

Performance Monitor

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• Example query to return all deprecated features usage– Many feature usages are actually SQL Server

internals and tools!

SELECT *

FROM sys.dm_os_performance_counters pc

WHERE object_name

LIKE '%Deprecated Features%'

AND cntr_value > 0

ORDER BY counter_name

Performance Monitor

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Performance Monitor

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• Querying for specific counters allows ad-hoc views of point-in-time information– But no graphs, charts, pretty colors

• For trend analysis, you have a few choices:– Store the query results in a table for later review

– Use PerfMon to store logs for later analysis

Performance Monitor

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• Some DBAs will store certain values in a table for later querying or for historical perspective– Watch the rest of the chapter prior to attempting

this!

-- Create a new table (”PerfCounters”)

-- and store current counters

SELECT *

INTO PerfCounters

FROM sys.dm_os_performance_counters

Performance Monitor

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• All DBAs will use PerfMon

Performance Monitor

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• Let’s take a look at specific objects and counters


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• PerfMon

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• There are a few specific areas you will want to monitor– Sometimes you can find that “Aha!” moment

using ad-hoc monitoring

– Sometimes you can only find that “Aha!” moment using trend analysis

Performance Monitor

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• A few topics before we go further:– PerfMon, counters with “/sec” in the name are

not cumulative

• Makes it easy to do ad-hoc monitoring

– PerfMon’s GUI defaults to taking measurements every one second

• Too often for trend analysis

• 30-seconds is generally “good enough” for ad-hoc troubleshooting

Performance Monitor

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• A few areas DBAs want to monitor:– Disk usage

– CPU usage

– Memory usage

• Let’s look at each!

Performance Monitor

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• A few areas DBAs want to monitor:– Disk usage

– CPU usage

– Memory usage

Performance Monitor

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• Disk usage monitoring – Disk I/O is commonly a performance stealer

– While SQL Server manages the “when” and “how often” of disk I/O, it is the operating system that performs I/O

– When monitoring disk activity, you need to monitor both SQL Server’s disk activity and the disk activity of the server as a whole

Disk Usage

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• Disk I/O cannot be isolated as a metric– Example: You are monitoring disk I/O and notice

that you have waits for disk reads/writes. Does this always mean that you need faster/more disks?

• No! Could be that you are paging constantly and memory is the real bottleneck

Disk Usage

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• Important system objects and counters:

Disk Usage

Object Counter When to worry

PhysicalDisk % Disk Time Consistently > 90%

PhysicalDisk Current Disk Queue Length

Particularly helpful when “% Disk Time” is high; allows to see how many I/O operations are waiting. Start worrying when this value is more than 1.5x the number of spindles

PhysicalDisk Avg. Disk Queue Length In many configs, a disk has 1 spindle. Start worrying when this value is more than 1.5x the number of spindles

Memory Page faults/sec “Some” page faults are normal; look for consistently high numbers

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• PhysicalDisk: % Disk Time– Tells us how “busy” your array is

– With this counter, you want to worry about sustained activity, not bursts

– A % Disk Time of more than 75% for several continuous minutes occurring several times a day can likely be resolved:

• By adding more disks to the array

• By using faster drives

Disk Usage

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• PhysicalDisk: % Disk Time (cont.)– When you do notice this is high, start looking at

the PhsyicalDisk: Current Disk Queue Length

• How many disks are in my array?– Tells you how many spindles you have

• How many I/O tasks are waiting?

– If the number of waiting tasks is 1.5 or higher per spindle, you need better I/O performance!

Disk Usage

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• PhysicalDisk: Avg. Disk Queue Length– Monitor this over time

• Business hours

• 24 hour period

• Peak times

– If you find that it exceeds more than 1.5 waiting tasks per spindle for continuous periods, increase disk performance

Disk Usage

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• Important SQL Server objects & counters to help diagnose I/O trouble– Object:

• SQL Server: Buffer Manager

– Counters:

• Page reads/sec

• Page writes/sec

Disk Usage

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• Page reads/sec and Page writes/sec measure physical reads, not logical reads– Not logical reads!

– Object:

• SQL Server: Buffer Manager

– Counters:

• Page reads/sec

• Page writes/sec

Disk Usage

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• High disk usage is not always a sign that you need more/faster disks– Sometimes poorly optimized queries can

masquerade as disk/memory/CPU issues

– Proper indexing and better queries can help eliminate slow queries as the culprit

One Caveat

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• PerfMon is great for monitoring disks directly attached to the server– Good but not great for monitoring SANs

– Check with your SAN vendor for suggestions on monitoring disk activity

A Final Caveat

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• Let’s play with PerfMon and look at Disk, CPU, and memory counters


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• Data Collector


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• While it is fun and educational to learn the various DMVs and DMFs along with the relationships between various system views, it is a huge time sink– Pro tip: download the SQL Server 2008 System

Views map

– http://learnitfirst.com/go.aspx?id=5GR5

Data Collector

http://learnitfirst.com/go.aspx?id=5GR5

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• How to be an unsuccessful DBA:– Try to remember all the DMVs, DMFs, and

Performance Counters

– Perform all of your performance data collection by running 100+ line queries daily

• Bonus points if you are the only person who knows where on the network these queries/files are located

– Spend 20 hours a week writing and running DMV-based queries

Data Collector

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• How to be an successful DBA:– Learn the most important DMVs, DMFs and

Performance Counters for your situation

• Every job is different and every database is different

• OLTP databases need different metrics than OLAP

• DBAs of a “single database only” server are often concerned with different things than DBAs of “20 database” servers

– Make your reporting easy to digest and easy to run

• No more 20 hours a week manually writing/running DMV-based queries!

Data Collector

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• Life as a DBA has traditionally been spent doing a lot of manual collection of performance data1. Create a tables to store performance data

2. Write the queries to collect the data

3. Create a job to store query results in tables

4. Write queries against tables to determine trends, etc

Data Collector

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• This was how things were done for more than a decade– As recent as SQL Server 2000…

– SQL Server 2005 debuted “Custom Reports” to help us

• Allowed integration of SSMS with Reporting Services to create professional quality reports based on our queries

• “SQL Server 2005 Performance Dashboard” reports

Data Collector

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• SQL Server 2008 includes the Data Collector to save you time– Creates a Management Data Warehouse (MDW)

that stores critical tuning, planning, and activity history

– Used daily by DBAs to do capacity planning, mitigate risks, free up time otherwise spent trying to write complex DMV/DMF queries!

Data Collector

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• The Data Collector is turned off by default– Enabling it is not a trivial decision…

– Performance data collection will impact your system

• It’s the Heisenberg uncertainty principle in action!

• How much of an impact depends on what you collect and how often

Data Collector

p. 131

• A disclaimer: – Please test, test, test this prior to deploying in a

production environment

– This is version 1.0 software

– Easy to “over collect”

• 30GB of data collection in three days!

Data Collector

p. 132

• Q& for Data Collector– “What type of information can DC collect?”

• Traces, PerfMon counters, SQL queries

– “Can I use DC to collect SQL 2005/2000 servers?”

• No, SQL Server 2008 only

– “Is this another one of those ‘Enterprise Edition only’ features?”

• No - works great in Standard and Workgroup Editions too!

Data Collector

p. 133

• Q& for Data Collector (cont.)– “How much of an impact will this have on my

server?”

• Microsoft says “5% of CPU and 250-350MB” for the built-in collection sets. Custom collection sets that you create may consume more

– More on this later…

Data Collector

p. 134

• Let’s learn a bit more about the Data Collector


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• The Data Collector (“DC”) is built into SSMS

Data Collector

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• You can enable the DC via the Configure Management Data Warehouse Wizard

Data Collector

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• Configuring the DC is a two-step process:1. Creating the Management Data Warehouse (the

“MDW”)

2. Setting up the actual collection of metrics

Data Collector

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“MDW”)


Data Collector

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• The MDW is a relational data warehouse to store all of your metrics for later analysis

Data Collector

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• When you create the MDW database, you define the name and physical file location

• Placing the MDW database on either a dedicated server or dedicated disks will minimize performance impact to monitored servers

Data Collector

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• The MDW data can be shared with non-sysadmins– All sysadmins have access

– Often want network admins to have access to performance data without having access to specific databases

Data Collector

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• There are three databases roles you can assign:

Data Collector

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• Once you decide security, you are ready to build the MDW

Data Collector

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“MDW”)



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“MDW”)


Data Collector

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• Back through the wizard…

Data Collector

p. 151

• The Cache directory is local storage on the monitored machine– Remember that this can be run from a central server

Data Collector

p. 152

• Data collector will create SQL Server jobs to perform data collection

• The job has two main components:1. Collect the info

2. Upload the info to the MDW

• Where you collect the info locally is the cache directory– Generally the %temp% folder

Data Collector

p. 153

• Ready, set, go!

Data Collector

p. 154

• What it should look like

Data Collector

p. 155

• Now comes the work…– You can use the System Data Collection Sets

– You can create your own collection sets

• There is no graphical way to create your own collection sets– Must write Transact-SQL

Data Collector

p. 156

• We’ll take a look at enabling the Data Collector and viewing the results


Documents

SQL Server Monitoring and Performance