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How Row versioning impact tempDB ?

Scenario: DBA found that tempdDBdatabase usage is getting high and most of the size is consumed by row versioning. DBA raised the issue with application team running that query. Once Application team close the session, tempDB usage comes normal.

Question: Application tea raise concern “How can a select statement on table consume huge amount of resources in tempDB?”

Answer:

As per BOL (https://technet.microsoft.com/en-us/library/ms175492%28v=sql.105%29.aspx) : Row versioning-based isolation levels (RCSI – Read Committed Snapshot Isolation) reduce the number of locks acquired by transaction by eliminating the use of shared locks on read operations. This increases system performance by reducing the resources used to manage locks. Performance is also increased by reducing the number of times a transaction is blocked by locks acquired by other transactions. Row versioning-based isolation levels increase the resources needed by data modifications.

While updating the row in a table or index, the new row is marked with a value called the transaction sequence number (XSN), XSN is unique number and keeps updating with upcoming new command \ transactions. When updating a row, New version is generated. If DB is already having previous version of this row, previous version of the row is stored in the version store and the new version of the row contains a pointer to the old version of the row in the version store. New row version stores corresponding XSN value for reflecting the time the row was modified.

All version stores are linked in a chain. SQL Server keep running clean up task to remove old versions which are not in use. As long as a transaction is open, all versions of rows that have been modified by that transaction must be kept in the version store. This is the reason due to which long running open transaction can cause huge tempDB row versioning space.

Here, Row X is getting updated by multiple transactions (Tv,Tx,Ty,Tz) and each time new version store is created. When DB engine tries to clear old unused version stores, it found open transaction using version X. DB engine not able to clear any row version from version store after version X. Due to which TempDB size keeps increasing due to row versioning.

–Check Size of tempDB consumed by Version Store

SELECT SUM(VERSION_STORE_RESERVED_PAGE_COUNT) AS [VERSION STORE PAGES USED],

(SUM(VERSION_STORE_RESERVED_PAGE_COUNT)*1.0/128) AS [VERSION STORE SPACE IN MB],

SUM(INTERNAL_OBJECT_RESERVED_PAGE_COUNT) AS [INTERNAL OBJECT PAGES USED],

(SUM(INTERNAL_OBJECT_RESERVED_PAGE_COUNT)*1.0/128) AS [INTERNAL OBJECT SPACE IN MB],

SUM(USER_OBJECT_RESERVED_PAGE_COUNT) AS [USER OBJECT PAGES USED],

(SUM(USER_OBJECT_RESERVED_PAGE_COUNT)*1.0/128) AS [USER OBJECT SPACE IN MB],

SUM(UNALLOCATED_EXTENT_PAGE_COUNT) AS [FREE PAGES],

(SUM(UNALLOCATED_EXTENT_PAGE_COUNT)*1.0/128) AS [FREE SPACE IN MB]

FROM SYS.DM_DB_FILE_SPACE_USAGE;

–Find session using version store

SELECT A.*,B.KPID,B.BLOCKED,B.LASTWAITTYPE,B.WAITRESOURCE,B.DBID,B.CPU,B.PHYSICAL_IO,B.MEMUSAGE,B.LOGIN_TIME,B.LAST_BATCH,

B.OPEN_TRAN,B.STATUS,B.HOSTNAME,B.PROGRAM_NAME,B.CMD,B.LOGINAME,REQUEST_ID

FROM SYS.DM_TRAN_ACTIVE_SNAPSHOT_DATABASE_TRANSACTIONS A

INNER JOIN SYS.SYSPROCESSES B

ON A.SESSION_ID = B.SPID

Reference: Rohit Garg (http://mssqlfun.com/)

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SQL Server Cluster LooksAlive and IsAlive Check

Windows Cluster Service use the “heartbeat” process to check whether each node is alive at both the OS level and SQL Server level. Heartbeat signals works on UDP.

Microsoft Windows Cluster service perform 2 health checks to perform node & resource availability check.

LooksAlive Health Check

1) LooksAlive is a quick lightweight health check

2) Run at interval of 5 Seconds

3) Does not impact performance but does not perform a thorough check

4) The check will succeed if the service appears to be running even though it might not be operational

5) If Fails calls “IsAlive” health check

6) Polling interval can be changed by adjusting LooksAlivePollInterval property of Cluster service

IsAlive Health Check

1) Run at interval of each 60 Second

2) Perform more detail check then LooksAlive

3) Run @@SERVERNAME to ensure that SQL Server is responding to queries

4) Does not ensure that all user databases are operational

5) Retired 5 times & then try to reconnect SQL Server

6) If all 5 retry fails, the SQL Server resource fails & Windows Cluster service will try to bring it online on other node as per configuration

7) Polling interval can be changed by adjusting IsAlivePollInterval property of Cluster service

Reference : Rohit Garg (http://mssqlfun.com/)

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Creation & Deletion of Database Snapshot by SQL Agent Job

Some days back, I received one query on my facebook page that user want to create Database Snapshot frequently & want to delete the oldest with same frequency. Quite interesting scenario. I did not go in deep why he has such requirement but I am clear that It can be done very easily.

So let’s do it. I have setup 2 scripts one for creation of snapshot & another is for deletion of snapshot. We can schedule a SQL agent jobs with 2 steps using these scripts.

Step 1 :-

DECLARE @CREATE_SS VARCHAR(MAX)

DECLARE @DT VARCHAR(100)

SET @DT = REPLACE(REPLACE(REPLACE(CONVERT(VARCHAR,GETDATE(),120),’-‘,’_’),’ ‘,’_’),’:’,’_’)

SET @CREATE_SS =

‘CREATE DATABASE TEST_’+@DT+’ ON

( NAME = TEST, FILENAME =

”C:\TEST_’+@DT+’.SS” )

AS SNAPSHOT OF TEST’

EXEC (@CREATE_SS)

Step 2 :-

IF(SELECT COUNT(*) FROM SYS.DATABASES WHERE SOURCE_DATABASE_ID = DB_ID(‘TEST’))>1

BEGIN

DECLARE @DROP_SS VARCHAR(MAX)

DECLARE @SS_NAME VARCHAR(100)

SELECT TOP 1 @SS_NAME = NAME FROM SYS.DATABASES WHERE SOURCE_DATABASE_ID = DB_ID(‘TEST’)

ORDER BY CREATE_DATE ASC

SET @DROP_SS = ‘DROP DATABASE ‘+ @SS_NAME

EXEC (@DROP_SS)

END

Reference : Rohit Garg (http://mssqlfun.com/)

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SQL Server 2005 || Database backup failed because Full Text catalog files are OFFLINE

Error : Database backup is getting fail with below reason.

The backup of full-text catalog ‘FullTextCatalog’ is not permitted because it is not online. Check errorlog file for the reason that full-text catalog becomes offline and bring it online. Or BACKUP can be performed by using the FILEGROUP or FILE clauses to restrict the selection to include only online data.

Cause : This behavior occurs because a full-text catalog in the database is not online. To perform a full backup, SQL Server 2005 requires all the database files and full-text catalogs in the database to be online.

Possible Reason : The database is attached from somewhere. However, you specify the incorrect location for the full-text catalog folder during the attachment.

Solution 1 (If FTS files are available but SQL server showing offline):

1. Locate the folder that contains the files for the problematic full-text catalog.

2. Run the ALTER DATABASE statement. Specify in the statement the correct location for the full-text catalog.

3. Rebuild the problematic full-text catalog in the database.

4. Perform a full backup of the database in SQL Server 2005 again.

Solution 2 (If FTS files are not available):

1. Detach database & attach again with all MDF, NDF, LDF & FTS files.

Web Reference :

1) http://support.microsoft.com/kb/923355

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Reference : Rohit Garg (http://mssqlfun.com/)

Renaming or Changing SQL Server Cluster Instance

Renaming or Changing SQL Server cluster instance process is bit different from renaming a stand-alone instance.

Refer : Rename or Change SQL Server Standalone Default Instance http://mssqlfun.com/2014/06/30/rename-or-change-sql-server-standalone-default-instance/)

Refer : Rename or Change SQL Server Standalone Named Instance (http://mssqlfun.com/2014/07/29/rename-or-change-sql-server-standalone-named-instance/)

The name of the virtual server is always the same as the name of the SQL Network Name (the SQL Virtual Server Network Name). You can change the name of the virtual server, you cannot change the instance name like in the case of standalone named instance. You can change a virtual SQL server named SQLServerinstance1 to some other name, such as SQLServerNewinstance1, but the instance part of the name, instance1, will remain unchanged.

Rename a virtual server :-

1. Using Cluster Administrator, change the SQL Network Name or SQL Server DNS Name to the new name.

2. Take the network name resource offline. This takes the SQL Server resource and other dependent resources offline as well.

3. Bring the SQL Server resource back online.

Verify the Renaming Operation :-

1) Using @@ServerName

2) Using sys.servers

Minimize network propagation delay :-

ipconfig /flushdns

ipconfig /registerdns

nbtstat –RR

You are done ! You should ask your application teams to reconnect with new Name or if they are using IP address then no changes required.

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Reference : Rohit Garg (http://mssqlfun.com/)

ERROR || The query processor is unable to produce a plan because the index ‘IND_TABLE’ on table or view ‘Table’ is disabled.

Table with clustered index is totally depended on index accessibility.

ERROR : The query processor is unable to produce a plan because the index ‘IND_TABLE’ on table or view ‘Table’ is disabled.

REASON : We find that some disable the cluster index due to which issue occur. Clustered index physically sort & save data in pages. When clustered index is disable, DB engine is not able to access data although data is available with table.

SCREENSHOT :

Note :

· There is no option to ENABLE the Index. You have to REBUILD or DROP & RECREATE it.

· This is not the case with non-clustered index.

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Reference : Rohit Garg(http://mssqlfun.com/)

SQL Server 2005 Onwards – Mirrored Backup || Cool Feature

SQL Server 2005 onwards includes the mirroring of backup media sets to provide redundancy of your critical database backups. Mirroring a media set increases backup reliability by reducing the impact of backup-device malfunctions. These malfunctions are very serious because backups are the last line of defense against data loss. SQL Server 2005 Standard Edition supports only a single backup copy during your backup operations. Depending on your requirements, SQL Server Enterprise allows you to create up to four mirrored media sets.

Benefits
Mirrored backup media sets improve availability by minimizing downtime during restore operations. A damaged backup could result in a longer restore time, or a restore failure. As databases grow, the probability increases that the failure of a backup device or media will make a backup unrestorable. Restoring a database is time sensitive, and mirrored backup media sets give you added protection to get your application fully functional more quickly.

In SQL Server 2005 and later versions, there is a Mirror command that makes a copy of the database backup to different locations while taking the original backup. The maximum limit of additional locations that can be specified with MIRROR clause is 3.

Mirrored backup can be taken in local computer system as well as in a local network. Let us now see two examples of mirror backup.

Example 1. Single File Backup to Multiple Locations using Mirror

BACKUP DATABASE AdventureWorks2012

TO DISK = ‘d:AdventureWorksBackup1.bak’

MIRROR TO DISK = ‘d:AdventureWorksBackupCopy.bak’

with format

If this command is being run for the first time, it is mandatory to use the WITH FORMAT clause; but for sub sequential runs it is not required. WITH FORMAT reinitializes the backup.

Example 2. Multiple File Backup to Multiple Locations using Mirror

BACKUP DATABASE AdventureWorks2012

TO DISK = ‘d:AdventureWorksBackup1.bak’,

disk = ‘d:AdventureWorksBackup2.bak’

MIRROR TO DISK = ‘d:AdventureWorksBackupCopy.bak’,

DISK = ‘d:AdventureWorksBackupCopy1.bak’

with format

Example 3. Only 4 Mirror are allowed (1 Backup file + 3 Mirror files)

Example 4. Different number of backup file & Mirror files – Error Occur

 

 

 

 

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Reference : Rohit Garg (http://mssqlfun.com/)

Query Execution Plan from XML to Graphical View

Convert Your SQL Server Query Execution Plan from XML to Graphical View

1) We can get queries execution plan from below query

SELECT

QS.*, CP.*

FROM SYS.DM_EXEC_QUERY_STATS AS QS

CROSS APPLY SYS.DM_EXEC_SQL_TEXT(SQL_HANDLE)AS ST

CROSS APPLY SYS.DM_EXEC_QUERY_PLAN(PLAN_HANDLE) AS CP

2) Column query_plan is having execution plan in XML form

3) When we click hyperlink in query_plan column, in SQL 2005,2008 & 2008 R2 it open as XML but in SQL 2012 it converted to graphical view

aaa

 

4) To convert XML to graphical view, save XML file with extension .sqlplan & open it with SSMS

5) Now plan will show in graphical view

 

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Reference : Rohit Garg (http://mssqlfun.com/)

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