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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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Restrict user to login from single Host

Restricting user to login from single host may be sometimes required from security point and other business requirements. We can achieve it through SERVER LEVEL LOGON TRIGGER.

ALTER TRIGGER TR_CHECK_LOGIN_TEST_HOST
ON ALL SERVER
FOR LOGON
AS
BEGIN

DECLARE @HOSTNAME VARCHAR(48)
DECLARE @PROGNAME VARCHAR(100)

SELECT @HOSTNAME = HOST_NAME FROM SYS.DM_EXEC_SESSIONS
WHERE SESSION_ID = @@SPID

IF ORIGINAL_LOGIN() = ‘TEST’ AND @HOSTNAME =’HostName’
ROLLBACK;

END

Once you create above trigger, Login “TEST” will be able to connect from specified host only. Connection from any other host will be failed and face below error: –

You can also find related errors in SQL Server Error log as well.

Error: 17892, Severity: 20, State: 1.

Logon failed for login ‘TEST’ due to trigger execution. [CLIENT: <local machine>]

Error: 3609, Severity: 16, State: 2.

The transaction ended in the trigger. The batch has been aborted.

 

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

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DMV-15 : Pending I/O requests……..sys.dm_io_pending_io_requests

sys.dm_io_pending_io_requests DMV (Dynamic Management View), described by BOL as follows: http://msdn.microsoft.com/en-us/library/ms188762.aspx

Returns a row for each pending I/O request in SQL Server.

It’s a very simple DMV used to see all pending I/O requests & there description.

You can combine this DMV with DMF – sys.dm_io_virtual_file_stats to see I/O pending requests with database files. You should run this query multiple times to check if the same files or drive letters consistently coming up on the top. If this is the case that means you facing I/O bottlenecks for that file or drive letter.

Query 1 : Details of I/O pending requests against each DB file

SELECT

DB_NAME(MF.DATABASE_ID) AS [DATABASE],

MF.PHYSICAL_NAME,

IPIR.IO_TYPE,

SUM(IPIR.IO_PENDING) TOTAL_PENDING_IO,

SUM(IPIR.IO_PENDING_MS_TICKS) TOTAL_PENDING_MS_TICKS,

SUM(VFS.NUM_OF_READS) TOTAL_READS,

SUM(VFS.NUM_OF_WRITES) TOTAL_WRITES

FROM

SYS.DM_IO_PENDING_IO_REQUESTS AS IPIR

INNER JOIN

SYS.DM_IO_VIRTUAL_FILE_STATS(NULL,NULL) AS VFS

ON IPIR.IO_HANDLE = VFS.FILE_HANDLE

INNER JOIN

SYS.MASTER_FILES AS MF

ON VFS.DATABASE_ID = MF.DATABASE_ID

AND VFS.FILE_ID = MF.FILE_ID

GROUP BY MF.DATABASE_ID, MF.PHYSICAL_NAME, IPIR.IO_TYPE

ORDER BY SUM(IPIR.IO_PENDING)

Remarks

1. To use this DMV, User required VIEW SERVER STATE permission on the server.

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

DMV-14 : Which Table has Max. rows & Size……..sys.dm_db_partition_stats

sys.dm_db_partition_stats DMV (Dynamic Management View), described by BOL as follows: http://msdn.microsoft.com/en-us/library/ms187737.aspx

Returns page and row-count information for every partition in the current database.

This DMV gives useful information about rowcount, free & reserved space of each table in database. It can be used for analysis purpose while adding new indexes, doing index Maintenance or checking the biggest table of database.

Query 1 : Details of table with row count & space used

SELECT

SS.NAME AS SCHEMANAME,

SO.NAME AS OBJECTNAME,

SI.NAME AS INDEXNAME,

PS.RESERVED_PAGE_COUNT*8 TOTAL_SPACE_CONSUMED_KB,

PS.USED_PAGE_COUNT*8 USED_SPACE_KB,

(PS.RESERVED_PAGE_COUNT – PS.USED_PAGE_COUNT)*8 FREE_SPACE_KB,

CASE

WHEN PS.INDEX_ID IN (0,1) THEN PS.ROW_COUNT

ELSE NULL

END AS ROW_COUNT

FROM

SYS.DM_DB_PARTITION_STATS PS

INNER JOIN SYS.OBJECTS SO ON SO.OBJECT_ID = PS.OBJECT_ID

INNER JOIN SYS.SCHEMAS SS ON SS.SCHEMA_ID = SO.SCHEMA_ID

LEFT JOIN SYS.INDEXES SI ON SI.OBJECT_ID = PS.OBJECT_ID

AND SI.INDEX_ID = PS.INDEX_ID

WHERE

SO.IS_MS_SHIPPED = 0

Sample Result

Remarks

1. To use this DMV, User required VIEW DATABASE STATE permission on the server.

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

DMV-13 : Finding locking & blocking……..sys.dm_tran_locks

sys.dm_tran_locks DMV (Dynamic Management View), described by BOL as follows: http://msdn.microsoft.com/en-us/library/ms190345.aspx

Returns information about currently active lock manager resources. Each row represents a currently active request to the lock manager for a lock that has been granted or is waiting to be granted.

The columns in the result set are divided into two main groups: resource and request. The resource group describes the resource on which the lock request is being made, and the request group describes the lock request.

This DMV is very useful in helping to identify locking and blocking issues on your data­base instances.

Query 1 : Details of currently active locks

SELECT

CASE DTL.REQUEST_SESSION_ID

WHEN -2 THEN ‘ORPHANED DISTRIBUTED TRANSACTION’

WHEN -3 THEN ‘DEFERRED RECOVERY TRANSACTION’

ELSE DTL.REQUEST_SESSION_ID END AS SPID,

DB_NAME(DTL.RESOURCE_DATABASE_ID) AS DATABASENAME,

SO.NAME AS LOCKEDOBJECTNAME,

DTL.RESOURCE_TYPE AS LOCKEDRESOURCE,

DTL.REQUEST_MODE AS LOCKTYPE,

ST.TEXT AS SQLSTATEMENTTEXT,

ES.LOGIN_NAME AS LOGINNAME,

ES.HOST_NAME AS HOSTNAME,

CASE TST.IS_USER_TRANSACTION

WHEN 0 THEN ‘SYSTEM TRANSACTION’

WHEN 1 THEN ‘USER TRANSACTION’ END AS USER_OR_SYSTEM_TRANSACTION,

AT.NAME AS TRANSACTIONNAME,

DTL.REQUEST_STATUS

FROM

SYS.DM_TRAN_LOCKS DTL

JOIN SYS.PARTITIONS SP ON SP.HOBT_ID = DTL.RESOURCE_ASSOCIATED_ENTITY_ID

JOIN SYS.OBJECTS SO ON SO.OBJECT_ID = SP.OBJECT_ID

JOIN SYS.DM_EXEC_SESSIONS ES ON ES.SESSION_ID = DTL.REQUEST_SESSION_ID

JOIN SYS.DM_TRAN_SESSION_TRANSACTIONS TST ON ES.SESSION_ID = TST.SESSION_ID

JOIN SYS.DM_TRAN_ACTIVE_TRANSACTIONS AT ON TST.TRANSACTION_ID = AT.TRANSACTION_ID

JOIN SYS.DM_EXEC_CONNECTIONS EC ON EC.SESSION_ID = ES.SESSION_ID

CROSS APPLY SYS.DM_EXEC_SQL_TEXT(EC.MOST_RECENT_SQL_HANDLE) AS ST

WHERE

RESOURCE_DATABASE_ID = DB_ID()

ORDER BY DTL.REQUEST_SESSION_ID

Sample Result

Remarks

1. Column request_session_id display the session ID owns the lock or raise request.

If value display is -2 indicates that the request belongs to an orphaned distributed transaction.

If value display is -3 indicates that the request belongs to a deferred recovery transaction, such as, a transaction for which a rollback has been deferred at recovery because the rollback could not be completed successfully.

2. Column request_owner_type details :-

· TRANSACTION = The request is owned by a transaction.

· CURSOR = The request is owned by a cursor.

· SESSION = The request is owned by a user session.

· SHARED_TRANSACTION_WORKSPACE = The request is owned by the shared part of the transaction workspace.

· EXCLUSIVE_TRANSACTION_WORKSPACE = The request is owned by the exclusive part of the transaction workspace.

· NOTIFICATION_OBJECT = The request is owned by an internal SQL Server component.

3. To use this DMV, User required VIEW SERVER STATE permission on the server.

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

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