Replicating Extra-Long Strings

Monica Rathbun walks us through a replication error:

Ever seen the below error? Until this week I hadn’t. So, I figured I’d take a little time and introduce it to those that had not.

Error Description: Length of LOB data (65754) to be replicated exceeds configured maximum 65536. Use the stored procedure sp_configure to increase the configured maximum value for max text repl size option, which defaults to 65536. A configured value of -1 indicates no limit

We ran into an issue with a customer this week, this error was flooding the error log. After a little digging I found it had to do with transactional replication (also applies to Change Data Capture) they had setup which included LOB data.

Read on to see what you can do to resolve this error.  Also, check out the comments and be glad you’re not in that boat…unless you are, in which case…

Smarter Distribution Database Cleanup

Sourabh Agarwal writes about an improvement to the Distribution Cleanup job:

In case of a highly active environment the cleanup job would need to delete a substantially large set of expired transactions and commands. The cleanup job uses a while loop to delete entries from the MSRepl_Commands and MSRepl_Transactions in batches of 2000 and 5000 rows, respectively. Depending on how large the MSRepl_Commands and MSRepl_Transactions Tables are, the cleanup activity can be cumbersome and time consuming, leading to several performance issues like lock blocking and sometimes replication agent failures. These hard-coded batch sizes work well when the size of the replication tables is small, but do not perform well when these replication tables become large, let’s say like 200-300 million rows.

The new implementation of the procedure allows users to parameterize the batch sizes inside the cleanup stored procedure and introduces a new adaptive approach to determine the batch sizes for each iteration. The procedure will, by default, honor any batch sizes that are mentioned explicitly as parameters. If no parameters are supplied, then the procedure will start with the default batch sizes of 2000 and 5000 rows and increase/decrease the batch sizes based on the performance of the previous iteration of the delete operation. If the time taken by the delete query improves by 50% compared to previous execution, the batch size value is increased by 20% up to a max value of 50000 rows per batch, and if the performance of the delete operation decreases by 20% as compared to the previous iteration, the batch size is decreased by 50% up to a default value of 2000/5000 rows per batch for MSReplCommands and MSRepl_Transactions, respectively.

It’s good to see Microsoft making incremental improvements to replication.

Putting The Distribution Database In An Availability Group

Sourabh Agarwal announces that you will soon be able to put the distribution database into an Availability Group:

Many enterprise customers have asked the capability to combine the usage of SQL Server replication and Always On, such that they can place replication distribution databases within an Always On AG to achieve high availability for their distribution databases, with the expectation that after doing so and when AG failover happens, SQL Server replication will continue functioning seamlessly and correctly. While the Replication publication and subscription databases can be configured to use Availability Groups, this support was lacking for the replication Distribution Databases.

SQL Server engineering team is excited to announce the new enhancement around the configuration of Replication Distribution Database in an Availability Group. This feature enhancement would be available with SQL Server 2017 CU6 and will be ported to SQL Server 2016 in a subsequent CU for SP2.

For those people using merge replication, it won’t be supported in this release.

Why CHECKDB Repair Invalidates Replication

Paul Randal explains why running DBCC CHECKDB on a published article will cause subscriptions to become invalidated:

Whenever I’m teaching and recovering from corruption, I always stress that if the REPAIR_ALLOW_DATA_LOSS option of DBCC CHECKDB (which I’ll just call ‘repair’ from now on) has to be used, you should do the following:

  • Run another DBCC CHECKDB to make sure that repair fixed everything and no new problems were exposed (by the repair fixing something that was blocking DBCC CHECKDB‘s access to a portion of the database)
  • Run DBCC CHECKCONSTRAINTS on any tables or indexes that were repaired and are involved in constraints (stated in Books Online too)
  • Reinitialize any replication subscriptions that are based on tables that were repaired (stated in Books Online too)
  • Reevaluate your HA/DR strategy so you don’t have to use repair again in future

One question I’m often asked is about why replication can be broken by running repair.

Read on for the answer.

Replication In Azure SQL DB Managed Instances

Tara Kizer looks at transactional replication in Azure SQL Database Managed Instances:

I have a love-hate relationship with replication. Mostly hate due to latency and errors, but it does serve its purpose. Before Availability Groups came out, I used Transactional Replication to copy data from the production OLTP database to another server so that we could offload reports.

Will it work with a Managed Instance?

My relationship is closer to hate-hate-love-hate-hate-love-hate.  Tara also provides some good advice about watching replication latency stats.

Replication Update: Dynamic Parameter Loading

Sourabh Agarwal announces a new feature with SQL Server replication:

When a new agent user profile is created and assigned to an agent, relevant entries are added to the [dbo].[MSagent_profiles] and the [dbo].[MSagent_parameters] tables in the MSDB database on the Distributor server. The values in the tables are updated whenever a parameter value is updated for the Profile. These values are reloaded from the tables, whenever a replication agent is restarted. As part of the improvement a new profile parameter “ProfileReloadInterval” with a default value of 300 seconds, was added to the profiles. This parameter dictates the interval at which the agent reloads the values from the tables. If using the default value, the agent will try and reload the values from the MSDB tables every 5 minutes. If it encounters that any of the above-mentioned parameter values have changed, the new values will be used by the agents.

I don’t work with replication enough to really see the benefit, but I’m sure somebody is giddy about this.

Tracking Replication Throughput

Phil Grayson has a couple of replication-related queries:

It’s a right laugh when replication goes wrong, if you haven’t been there and use replication heavily, you’re in for the night / early morning of your life. Like any issue, the cause of replication problems could be one of many, but checking what commands are being passed through the distributor is always worthwhile, as an inefficient query could be updating the same data over and over, putting unnecessary strain on the distributor, infrastructure or the subscribers.

Read on for those queries.

Transactional Replication And SQL On Linux

Phil Grayson shows a way to get transactional replication working on Linux:

Microsoft have stated that transactional replication isn’t supported on Linux and we’re not sure if they intend to in the future. This means that if you try to add the server to a publisher, you get the following message. So you can’t use the GUI and it also means that you can’t use pull as the necessary files won’t be there.

Despite that warning, there is a way to set up a push subscription; click through for that way.

Important Stored Procedures For Transactional Replication

Drew Furgiuele (who always starts off with the red ring) explains three important replication stored procedures:

We all have that one scenario where replication caused us some major headaches. It’s through these headaches, though, that the search for better ways to handle problems comes up. Through my time with replication, I’ve researched and used a lot of system tables, views, and stored procedures to diagnose or fix replication problems. There’s actually a lot of things you can get into if you’re so inclined. If you put me on the spot though, and asked me “Drew, what do you think are the most helpful replication internal objects to know?” I’d tell you I have three, and they’re all stored procedures.

Here then, in no particular order, are three system stored procedures that I feel any DBA that has to deal with transactional replication should at least be aware of. They each have a unique purpose, and each can be used to quickly monitor, fix, or even destroy replication forcibly, respectively.

Read the whole thing.

Transactional Replication And Temporal Tables

Transactional replication found Drew Furgiuele’s little black book of T-SQL syntax and went ballistic when it found out Drew was seeing temporal tables on the side:

So let’s say you ran this script (or, maybe someone checked it in as a database change to production). For a while, things are great: you’re making changes to data on your publisher and things are flowing nicely to your subscribers. Sooner or later though, someone’s going to ask you to set up a new subscription (or maybe you need to reinitialize one). Let’s simulate that on my lab: we’re going to remove Person.Address from replication and we’re going to put it back, and then create a snapshot. The key difference here is that now, Person.Address has system versioning turned on. When we try and add the table back to the publication, we’re in for a shock:

This could come back to bite you, so if you use replication and are interested in temporal tables, read this closely.

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