Transactional Replication – Understanding Latency

By

Abhay ChaudharyDatabase Architect (IBM India Pvt.Ltd.)

MCTS\MCITP : SQL Server 2005 , SQL Server 2008, SQL Server 2008 BI ,MCTS: SQL Server 2008 DB Developer , OCP 9i

9+ years of Database Management experience .Ex- Microsoft PSS (2007-2009).Blogging on SQL Server articles since 2008.Assist SQL Server technical community through various forums.

Abhay Chaudhary

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Latency

Where is the latency ?

How much is the latency ?

Few questions you need to answer

Finding threads involved in latency –option 1

Finding threads involved in latency –option 2 (prefer)

Reasons for Log Reader-Reader thread latency

Reasons for Log Reader-writer thread latency

Reasons for Dist Agent-Reader thread latency

Reasons for Dist Agent-Writer thread latency

References

Agenda

Slowness of transaction delivery by either Log reader agent or Distribution agent or both .

For log reader agent its slowness in picking up the transactions that are marked for replication in Publisher log.

For Distribution agent it’s the slowness in picking up the transactions from the distribution database .

Latency

Where is the latency ?

If all the subscriptions of one publisher are affected then the latency is from publisher to distributor .If a few subscriptions of one publisher are affected then the latency is from distributor to subscriber .

Query MSLogreader_history and MSDistribution_history system table in Distribution agents.

Example : select * from MSlogreader_history where order by

time desc select * from MSdistribution_history where order by

time desc

What's next ?

How much is the latency ?

Is the latency in reading the publisher log ?

Is the latency in writing the data to the distributor ?

Is the latency reading the data from distributor database ?

Is the latency in writing the data to the subscriber tables ?

Few questions you need to answer

Check the latest comments section of MSlogreader_history and MSdistribution_history tables in distribution DB.

Log reader

Check the latest comments section of MSDistribution_history and MSdistribution_history tables in distribution DB.

Distribution agent

State 1 = Normal

State 2 = Reader Thread waits for Writer Thread

State 3 = Writer thread waits for Reader thread

Finding threads involved in latency –option 1

Stop the Distributor agent and check the job history .

Finding threads involved in latency –option 2

Stop the Distributor agent and check the job history .

Finding threads involved in latency –option 2 ……

Most common reasons are :

Big T-log size due to maintenance . Very likely

large Batch(s) of replicated transactions . Very likely

Storage latency (Slow Read IO)

High CPU \IO or load on the Server in due course of time . likely

Reasons for Log Reader -Reader thread latency

Most common reasons are : Blocking on MSRepl_Transactions and MSRepl_Commands tables. High IO\CPU load on the Dist server . likely No \less maintenance on MSRepl_Transactions and

MSRepl_Commands tables. Very likely Storage Latency (Slow writes) Network Latency likely

Reasons for Log Reader –Writer thread latency

Most common reasons are :

Large Batch of transactions Very likely

Lack of maintenance (Fragmentation and stale stats ) Very likely

Keeping the publication snapshots likely

Long execution of cleanup job causing latency likely

Slow Storage (Read latency )

Reasons for Dist Agent -Reader thread latency

Most common reasons are : Resource consuming queries running already Very likely Lack of maintenance Very likely Network issues Storage issues Too many indexes Very likely Blocking Very likely High CPU Very likely

Reasons for Dist Agent –Writer thread latency

How Transactional Replication Workshttp://msdn.microsoft.com/en-us/library/ms151706.aspx

Physical Architecture (Replication)http://msdn.microsoft.com/en-us/library/bb500345.aspx

Transactional Replication Overviewhttp://msdn.microsoft.com/en-us/library/ms151176.aspx

sp_replcmds (Transact-SQL)http://msdn.microsoft.com/en-us/library/ms186983.aspx

sp_repldone (Transact-SQL)http://msdn.microsoft.com/en-us/library/ms173775.aspx

Solution Design Considerations (Replication)http://msdn.microsoft.com/en-us/library/ms152508.aspx

Designing and Implementing (Replication)http://msdn.microsoft.com/en-us/library/ms151847.aspx

Replication Agents Overviewhttp://msdn.microsoft.com/en-us/library/ms152501.aspx

References

Q & A

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