In part 1 of the A wait a day series, we discussed the self induced WAITFOR statement. Before discussing further waits, I
thought it would be helpful to review the sys.dm_os_wait_stats dmv.
The sys.dm_os_wait_stats dmv returns details about waits that threads have encountered. The DMV returns five columns,
wait_type, waiting_tasks_count, wait_time_ms, max_wait_time_ms and signal_wait_time_ms.
In general, there are three types of waits: Resource waits, External waits and Queue waits.
Resource waits include locks, network, latches and disk waits.
Queue waits tend to be seen when a worker is waiting for work or if the worker is idle.
External waits happen when SQL Server is waiting for an external even to complete.
Saturday, October 29, 2011
Friday, October 28, 2011
A wait a day: WAITFOR
This is part 1 of a series dedicated to SQL Server Waits. Part 1 will cover the WAITFOR statement, which does not introduce a wait, but I thought we could cover the statement.
The WAITFOR statement will block the processing of a batch, stored procedure or transaction based on the time or interval defined with the command.
The statement below waits 1 minute .
WAITFOR DELAY '00:01:00'
This statement below waits until 11:02 AM local time
WAITFOR TIME '11:02:00'
Example
DECLARE @HireTime Char(8)
SET @HireTime = '12:00:00'
--Select all employees in the system as of noon today
WAITFOR TIME @HireTime
SELECT * FROM AdventureWorks.HumanResources.Employee
If you have set the @HireTime variable far enough into the future, you can run the statement below to see what's happening behind the scenes.
SELECT r.Wait_type, r.wait_time, r.last_wait_type, r.open_transaction_count, t.text as SQLStatement
FROM sys.dm_exec_requests r .
cross apply sys.dm_exec_sql_text(r.Plan_handle) t.
WHERE session_id > 50
Be cautious when using the WAITFOR Statement as SQL Server uses a seperate thread for each WAITFOR occurance.
You cannot use WAITFOR in view definitions or when opening a CURSOR.
The WAITFOR statement will block the processing of a batch, stored procedure or transaction based on the time or interval defined with the command.
The statement below waits 1 minute .
WAITFOR DELAY '00:01:00'
This statement below waits until 11:02 AM local time
WAITFOR TIME '11:02:00'
Example
DECLARE @HireTime Char(8)
SET @HireTime = '12:00:00'
--Select all employees in the system as of noon today
WAITFOR TIME @HireTime
SELECT * FROM AdventureWorks.HumanResources.Employee
If you have set the @HireTime variable far enough into the future, you can run the statement below to see what's happening behind the scenes.
SELECT r.Wait_type, r.wait_time, r.last_wait_type, r.open_transaction_count, t.text as SQLStatement
FROM sys.dm_exec_requests r .
cross apply sys.dm_exec_sql_text(r.Plan_handle) t.
WHERE session_id > 50
Be cautious when using the WAITFOR Statement as SQL Server uses a seperate thread for each WAITFOR occurance.
You cannot use WAITFOR in view definitions or when opening a CURSOR.
Tuesday, October 25, 2011
Disk Latency SQL Script
Below is a script that I use to analyze disk latency and read/write ratios for my database server disks. The script is a combination of a couple of scripts from two different sources.
I'm borrowing the latency code from Listing 7-7 from Professional SQL Server 2008 Internals and Troubleshooting by Wrox Press. I'm borrowing the read/write ratios from Listing 2.1 from SQL Server Hardware from Redgate Books.
I added in some additional columns for drive letters, file extensions and disk latency thresholds.
I find the output very useful in determining if my storage system is keeping up with my SQL Server.
SELECT DISTINCT DB_NAME(Database_id) AS DB, FILE_ID,
io_stall_read_ms / num_of_reads as 'AVG read Transfer/ms',
CASE WHEN b.groupid = 1 THEN
CASE WHEN io_stall_read_ms / num_of_reads < 10 THEN 'Target'
WHEN io_stall_read_ms / num_of_reads between 10 and 20 THEN 'Acceptable'
WHEN io_stall_read_ms / num_of_reads > 20 THEN 'Unacceptable'
ELSE 'N/A' END
ELSE 'N/A' END AS ReadRange,
CAST(100. * num_of_reads / ( num_of_reads + num_of_writes ) AS DECIMAL(10,1)) AS [# Reads Pct] ,
CAST(100. * num_of_bytes_read / ( num_of_bytes_read + num_of_bytes_written ) AS DECIMAL(10,1)) AS [Read Bytes Pct] ,
io_stall_write_ms / num_of_writes AS 'AVG write Transfer/ms',
CASE WHEN b.groupid = 0 THEN
CASE WHEN io_stall_write_ms / num_of_writes < 10 THEN 'Target'
WHEN io_stall_write_ms / num_of_writes between 10 and 20 THEN 'Acceptable'
WHEN io_stall_write_ms / num_of_writes > 20 THEN 'Unacceptable'
ELSE 'N/A' END
ELSE 'N/A' END AS WriteRange,
CAST(100. * num_of_writes / ( num_of_reads + num_of_writes ) AS DECIMAL(10,1)) AS [# Write Pct] ,
CAST(100. * num_of_bytes_written / ( num_of_bytes_read + num_of_bytes_written ) AS DECIMAL(10,1)) AS [Written Bytes Pct],
LEFT(b.filename,3) AS DriveLetter, RIGHT(b.filename,3) AS FileExtension
FROM sys.dm_io_virtual_file_stats (-1,-1) a
INNER JOIN sys.sysaltfiles b on a.Database_id = b.dbid and a.file_id = b.fileid
WHERE num_of_reads > 0
AND num_of_writes > 0
I'm borrowing the latency code from Listing 7-7 from Professional SQL Server 2008 Internals and Troubleshooting by Wrox Press. I'm borrowing the read/write ratios from Listing 2.1 from SQL Server Hardware from Redgate Books.
I added in some additional columns for drive letters, file extensions and disk latency thresholds.
I find the output very useful in determining if my storage system is keeping up with my SQL Server.
SELECT DISTINCT DB_NAME(Database_id) AS DB, FILE_ID,
io_stall_read_ms / num_of_reads as 'AVG read Transfer/ms',
CASE WHEN b.groupid = 1 THEN
CASE WHEN io_stall_read_ms / num_of_reads < 10 THEN 'Target'
WHEN io_stall_read_ms / num_of_reads between 10 and 20 THEN 'Acceptable'
WHEN io_stall_read_ms / num_of_reads > 20 THEN 'Unacceptable'
ELSE 'N/A' END
ELSE 'N/A' END AS ReadRange,
CAST(100. * num_of_reads / ( num_of_reads + num_of_writes ) AS DECIMAL(10,1)) AS [# Reads Pct] ,
CAST(100. * num_of_bytes_read / ( num_of_bytes_read + num_of_bytes_written ) AS DECIMAL(10,1)) AS [Read Bytes Pct] ,
io_stall_write_ms / num_of_writes AS 'AVG write Transfer/ms',
CASE WHEN b.groupid = 0 THEN
CASE WHEN io_stall_write_ms / num_of_writes < 10 THEN 'Target'
WHEN io_stall_write_ms / num_of_writes between 10 and 20 THEN 'Acceptable'
WHEN io_stall_write_ms / num_of_writes > 20 THEN 'Unacceptable'
ELSE 'N/A' END
ELSE 'N/A' END AS WriteRange,
CAST(100. * num_of_writes / ( num_of_reads + num_of_writes ) AS DECIMAL(10,1)) AS [# Write Pct] ,
CAST(100. * num_of_bytes_written / ( num_of_bytes_read + num_of_bytes_written ) AS DECIMAL(10,1)) AS [Written Bytes Pct],
LEFT(b.filename,3) AS DriveLetter, RIGHT(b.filename,3) AS FileExtension
FROM sys.dm_io_virtual_file_stats (-1,-1) a
INNER JOIN sys.sysaltfiles b on a.Database_id = b.dbid and a.file_id = b.fileid
WHERE num_of_reads > 0
AND num_of_writes > 0
Thursday, October 20, 2011
Date formatting
I always feel like I'm digging around to find a list of date formats and what their outputs look like, so I've included a list below. I'm not going to discuss how each format breaks down the date, but I've included the formatting code along with the output. I may have missed a few, but the majority I have used in the past are listed below.
| format code | output |
| default | 2011-10-20 06:59:22.590 |
| convert(varchar,getDate(),100) | Oct 20 2011 6:59AM |
| convert(varchar,getDate(),101) | 10/20/2011 |
| convert(varchar,getDate(),102) | 2011.10.20 |
| convert(varchar,getDate(),103) | 20/10/2011 |
| convert(varchar,getDate(),104) | 20.10.2011 |
| convert(varchar,getDate(),105) | 20-10-2011 |
| convert(varchar,getDate(),106) | 20 Oct 2011 |
| convert(varchar,getDate(),107) | Oct 20, 2011 |
| convert(varchar,getDate(),108) | 06:59:00 |
| convert(varchar,getDate(),109) | Oct 20 2011 6:59:00:553AM |
| convert(varchar,getDate(),110) | 10-20-2011 |
| convert(varchar,getDate(),111) | 2011/10/20 |
| convert(varchar,getDate(),112) | 20111020 |
| convert(varchar,getDate(),113) | 20 Oct 2011 06:59:00:553 |
| convert(varchar,getDate(),114) | 06:59:00:553 |
| convert(varchar,getDate(),120) | 2011-10-20 06:59:00 |
| convert(varchar,getDate(),121) | 2011-10-20 06:59:00.553 |
| convert(varchar,getDate(),126) | 2011-10-20T06:59:00.553 |
| convert(varchar,getDate(),127) | 2011-10-20T06:59:00.553 |
| convert(varchar,getDate(),130) | 23 ?? ?????? 1432 6:59:00:553 |
| convert(varchar,getDate(),131) | 23/11/1432 6:59:00:553AM |
Wednesday, October 19, 2011
Free Ebook: SQL Server Hardware Choices Made Easy
Below is a free ebook available on Red Gate's site. SQL Server Hardware Choices Made Easy was written by Glenn Berry.
http://www.red-gate.com/products/dba/sql-virtual-restore/entrypage/sql-server-hardware-ebook?utm_source=forg&utm_medium=email&utm_content=glennberry_ebook20110825&utm_campaign=sqlvirtualrestore
For those of you who like the free ebook, I strongly suggest you buy his most recent hard copy book, SQL Server Hardware. http://www.amazon.com/SQL-Server-Hardware-Glenn-Berry/dp/1906434638
http://www.red-gate.com/products/dba/sql-virtual-restore/entrypage/sql-server-hardware-ebook?utm_source=forg&utm_medium=email&utm_content=glennberry_ebook20110825&utm_campaign=sqlvirtualrestore
For those of you who like the free ebook, I strongly suggest you buy his most recent hard copy book, SQL Server Hardware. http://www.amazon.com/SQL-Server-Hardware-Glenn-Berry/dp/1906434638
Free Ebook: Intro to Denali
The link below will take you to the place where you can download a free ebook that covers Denali, which is now called SQL Server 2012. This book is similar to the free ebook that was available for SQL Server 2008 R2.
http://blogs.msdn.com/b/microsoft_press/archive/2011/10/11/free-ebook-introducing-microsoft-sql-server-code-name-denali-draft-preview.aspx
http://blogs.msdn.com/b/microsoft_press/archive/2011/10/11/free-ebook-introducing-microsoft-sql-server-code-name-denali-draft-preview.aspx
Monday, October 17, 2011
Detecting the version of SQL Server
I always find myself using google to remember how to check for the exact version of SQL Server that I'm working with. @@Version always makes it more difficult to parse with everything thrown into a single column, so prefer a different method.
Below is a script that I like to Use. This is really just a combination of Method 3 and Method 4 from the following link:
http://support.microsoft.com/kb/321185
SELECT @@Version as FullDetail, SERVERPROPERTY('productversion') as VersionNumber, SERVERPROPERTY ('productlevel') as ServicePack, SERVERPROPERTY ('edition') as Edition
The above method is the way I teach people to do it, because I've seen a few Junior DBA's confuse the OS information as SQL Server info from some of the details in @@Version.
Below is a script that I like to Use. This is really just a combination of Method 3 and Method 4 from the following link:
http://support.microsoft.com/kb/321185
SELECT @@Version as FullDetail, SERVERPROPERTY('productversion') as VersionNumber, SERVERPROPERTY ('productlevel') as ServicePack, SERVERPROPERTY ('edition') as Edition
The above method is the way I teach people to do it, because I've seen a few Junior DBA's confuse the OS information as SQL Server info from some of the details in @@Version.
Saturday, October 15, 2011
SQL Server 2012
Although I did not attend PASS this year, one of the announcements was the next version of SQL Server (Denali) will be called SQL Server 2012. Like I said, I didn't make it to the summit this year, but below are some links containing recents news about SQL Server 2012.
http://www.informationweek.com/news/software/info_management/231900633
http://www.readwriteweb.com/enterprise/2011/10/sql-server-2012-microsofts-que.php
http://www.microsoft.com/sqlserver/en/us/future-editions.aspx
Please comment on this post if you have additional details or links to share. Thanks!
http://www.informationweek.com/news/software/info_management/231900633
http://www.readwriteweb.com/enterprise/2011/10/sql-server-2012-microsofts-que.php
http://www.microsoft.com/sqlserver/en/us/future-editions.aspx
Please comment on this post if you have additional details or links to share. Thanks!
SQL Server 2008 SP3
Microsoft announced last week that Service Pack 3 is available for SQL Server 2008. SP2 is not a prerequisite. SP3 contains cumulative updates of SQL Server 2008 SP2 cumulative update package 1 to 4
Details about the contents of SP3 can be found below.
http://www.microsoft.com/download/en/details.aspx?id=27594
Details about the contents of SP3 can be found below.
http://www.microsoft.com/download/en/details.aspx?id=27594
Friday, October 14, 2011
How to optimize SQL Server for Ad-hoc workloads
I'm not going to get into the debate of whether Stored procedures are better than ad-hoc queries. There have been plenty of well written articles supporting each concept. I will stop short and simply say that they both work well under certain conditions.
I want to dive into the affects of using ad-hoc queries on SQL server. These aren't necessarily ill affects, but I want to go over how the SQL Server cache is impacted with ad-hoc queries and how you can tune SQL Server if your workload is primary ad-hoc in nature.
I said I wouldn't get into debate of whether Stored procedures are better than ad-hoc queries, but I think it's important to discuss common circumstances a SQL server would be under an ad-hoc load.
1. There are several reporting engines in Enterprise applications that allow users to choose an infinite amount of columns, groupings, and custom calculated columns. It would be difficult for stored procedures to handle this abstraction.
2. ORM Frameworks such as Hibernate and LINQ generate allow you to generate SQL within the Object-oriented layer. The goal of these types of frameworks are to allow the object layer to generate the sql necessary to interface with the database.
3. Analytical and Investigative queries handling one-off questions related to support and or debugging.
Now that you have a background on why ad-hoc queries might be used, I will share a couple of sql statements that will inform you if your SQL Server is under an ad-hoc load.
First let's take a look at the dynamic management view, sys.dm_exec_cached_plans. This DMV returns information about each cached plan, how many times it was executed, the plan size and so forth ...
select * FROM sys.dm_exec_cached_plans where objtype = 'Adhoc'
Take special note of the size of your adhoc plans. I will reference the size of plans later in this post.
Below are the types of objects that can have cached plans.
Proc = Stored procedure, Prepared = Prepared statement, Adhoc = Ad ho
c query,ReplProc = Replication filter procedure, Trigger,View, Default, UsrTab = User table, SysTab = System table, Check = CHECK constraint and Rule
You will notice in the sample output from my query, that the use counts are relatively low for the cached plans on my environment. This is an important concept to think about when planning for ad-hoc queries. Again, there is nothing wrong with ad-hoc queries, I am simply suggesting that ah-hoc queries have low plan re-usage when compared to Stored Procedures, and this can affect cache storage in SQL Server.
You may be asking yourself, my database server has plenty of memory, so what's the big deal?
If you are experiencing large amounts of one time ad-hoc query statements, the procedure cache will start to fill with the full plans and for ad-hoc statements that may never be run again. This is wasteful. If the cache is more than 25% full of these one time ad-hoc plans, then you may be able to take advantage of a newer feature enabled in SQL Server 2008, "Optimize for Ad hoc workloads".
Below is a SQL statement that will determine your current cache size.
select SUM (CONVERT(decimal(14,4),size_in_bytes)) / 1048576 as CurrentCacheSize_MB
FROM sys.dm_exec_cached_plans
This next statement will help you determine the amount of single use ad-hoc plans
select SUM(CONVERT(decimal(14,4), size_in_bytes)) / 1048576 as CurrentAdHocSize_MB
FROM sys.dm_exec_cached_plans
WHERE usecounts = 1 AND objtype = 'adhoc'
If your ratio is more than 25 %, then you should consider enabling the "Optimize for Ad hoc workloads" feature.
The Optimize for Ad hoc workloads feature allows for just the stubs of the query plans to be cached, instead of the full plan. The stub is small, around 18 bytes, and this allows for all plans to remain in the cache for a longer periods of time. The goal is to keep plans who have higher re-use counts in the cache longer. Creating execution plans is a very expensive operation for SQL Server, so the server's goal is to re-use existing plans whenever possible.
Look at my query results again, you will notice the difference in size of the plan that is being stored.
select * FROM sys.dm_exec_cached_plans where objtype = 'Adhoc'
A quick recap.
Think about the types of applications that are using your database. Try looking at the sys.dm_exec_cached_plans to understand if the query plans in your SQL Server cache are being re-used and what type of plans are in the cache. You may be surprised to learn how your database is being utilized by the SQL statements that are run against it.
Enjoy
I want to dive into the affects of using ad-hoc queries on SQL server. These aren't necessarily ill affects, but I want to go over how the SQL Server cache is impacted with ad-hoc queries and how you can tune SQL Server if your workload is primary ad-hoc in nature.
I said I wouldn't get into debate of whether Stored procedures are better than ad-hoc queries, but I think it's important to discuss common circumstances a SQL server would be under an ad-hoc load.
1. There are several reporting engines in Enterprise applications that allow users to choose an infinite amount of columns, groupings, and custom calculated columns. It would be difficult for stored procedures to handle this abstraction.
2. ORM Frameworks such as Hibernate and LINQ generate allow you to generate SQL within the Object-oriented layer. The goal of these types of frameworks are to allow the object layer to generate the sql necessary to interface with the database.
3. Analytical and Investigative queries handling one-off questions related to support and or debugging.
Now that you have a background on why ad-hoc queries might be used, I will share a couple of sql statements that will inform you if your SQL Server is under an ad-hoc load.
First let's take a look at the dynamic management view, sys.dm_exec_cached_plans. This DMV returns information about each cached plan, how many times it was executed, the plan size and so forth ...
select * FROM sys.dm_exec_cached_plans where objtype = 'Adhoc'
Take special note of the size of your adhoc plans. I will reference the size of plans later in this post.
Below are the types of objects that can have cached plans.
Proc = Stored procedure, Prepared = Prepared statement, Adhoc = Ad ho
c query,ReplProc = Replication filter procedure, Trigger,View, Default, UsrTab = User table, SysTab = System table, Check = CHECK constraint and Rule
You will notice in the sample output from my query, that the use counts are relatively low for the cached plans on my environment. This is an important concept to think about when planning for ad-hoc queries. Again, there is nothing wrong with ad-hoc queries, I am simply suggesting that ah-hoc queries have low plan re-usage when compared to Stored Procedures, and this can affect cache storage in SQL Server.
You may be asking yourself, my database server has plenty of memory, so what's the big deal?
If you are experiencing large amounts of one time ad-hoc query statements, the procedure cache will start to fill with the full plans and for ad-hoc statements that may never be run again. This is wasteful. If the cache is more than 25% full of these one time ad-hoc plans, then you may be able to take advantage of a newer feature enabled in SQL Server 2008, "Optimize for Ad hoc workloads".
Below is a SQL statement that will determine your current cache size.
select SUM (CONVERT(decimal(14,4),size_in_bytes)) / 1048576 as CurrentCacheSize_MB
FROM sys.dm_exec_cached_plans
This next statement will help you determine the amount of single use ad-hoc plans
select SUM(CONVERT(decimal(14,4), size_in_bytes)) / 1048576 as CurrentAdHocSize_MB
FROM sys.dm_exec_cached_plans
WHERE usecounts = 1 AND objtype = 'adhoc'
If your ratio is more than 25 %, then you should consider enabling the "Optimize for Ad hoc workloads" feature.
The Optimize for Ad hoc workloads feature allows for just the stubs of the query plans to be cached, instead of the full plan. The stub is small, around 18 bytes, and this allows for all plans to remain in the cache for a longer periods of time. The goal is to keep plans who have higher re-use counts in the cache longer. Creating execution plans is a very expensive operation for SQL Server, so the server's goal is to re-use existing plans whenever possible.
Look at my query results again, you will notice the difference in size of the plan that is being stored.
select * FROM sys.dm_exec_cached_plans where objtype = 'Adhoc'
A quick recap.
Think about the types of applications that are using your database. Try looking at the sys.dm_exec_cached_plans to understand if the query plans in your SQL Server cache are being re-used and what type of plans are in the cache. You may be surprised to learn how your database is being utilized by the SQL statements that are run against it.
Enjoy
Thursday, October 13, 2011
Mixing Editions within a single Instance
The other day I inherited a SQL Server 2008 R2 Standard Server. I was tasked with adding the Enterprise version of SQL Server Analysis Services. To be honest, I was not sure if you could mix editions of SQL Server within the same instance. It's important to note that when I refer to edition, I'm not referring to version (2005, 2008, R2), I'm referring to Developer, Standard or Enterprise.
I initiated the installation process and soon found the answer to my question. If you look at the screen shot below, you will notice the installation wizard warns you about mixing editions within the same instance.
It's also important to note that features are different than shared services. Shared services are applications like SQL Server Management Studio and Integration Services. Features are the Database Engine, Report Services and Analysis Services.
I hope you enjoyed this post, happy installing.
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