Availability Group Tips

Derik Hammer has some tips to help you learn about Availability Groups:

3. Use MultiSubnetFailover=true

The Availability Group Listener is technically an optional component of an Availability Group. However, in my opinion it is necessary. By default, your listener will register all IP addresses as DNS A records and it will have multiple IP addresses when your cluster crosses subnets, most commonly when you have disaster recovery between data centers. Using the MultiSubnetFailover=true parameter in your client connection strings will attempt to connect to all IP addresses and completes the connection on the first thread to succeed. The listener ensures that only one IP address is online at a time, therefore you always connect to correct node.

This feature effectively bypasses the limitations of your DNS cache. Traditionally, you would cache the IP address for a DNS record. When you needed the client to connect to a different IP address using the same virtual network name, you would have to wait for the time to live setting to expire. This would delay your recovery time. With the MultiSubnetFailover setting, you can still cache your IP addresses but without the delay that they could induce.

There’s some good reading here.

Perfmon Counters For Monitoring AGs

Tracy Boggiano has a set of Perfmon counters she uses to monitor Availability Groups:

Monitoring Availability Groups can be tricky.  The DMVs don’t update the log_send_rate and redo_rate all the time especially if nothing is happening so if you try to use those for calculations when monitoring you could false results worse yet pages in the middle of the night.  In order to calculate the log_send_rate and redo_rate you need to capture the perfmon counters ‘Log Bytes Flushed/sec’, ‘Redone Bytes/sec’, and ‘Log Bytes Received/sec’ into temporary tables WAITFOR 1 second then capture them again.  Below is query that captures this along side what you see in the DMV for when Microsoft might fix the issue.

Click through for a script as well as a Powershell cmdlet wrapper for running against a set of hosts in your Central Management Server.

Troubleshooting AG Creation Failure

Anthony Nocentino digs into logs to troubleshoot a failure when trying to create an Availability Group:

Now we have some data to look through!

When we look at the contents of the cluster logs, we’re totally on the other side of the spectrum when it comes to information verbosity. The logs so far have been pretty terse and haven’t really told us about what’s causing the failure…well dig through this log and you’ll likely find your reason and a lot more information. Good stuff to look at to get an understanding of the internals of WSFCs. Now for the the reason my Availability Group creation failed was permissions. Check out the log entries.

It’s a good troubleshooting guide.

Recompiling Memory-Optimized Procedures On AGs

Ned Otter takes note of how natively compiled stored procedures differ from traditional stored procedures, especially on Availability Group secondary nodes:

As of SQL 2016, the database engine automatically updates statistics for memory-optimized tables (documentation here), but recompilation of native modules must still be performed manually. But hey, that’s way better than SQL 2014, when you couldn’t recompile at all; you had to drop/recreate the native module. And natively compiled stored procedures don’t reside in the plan cache, because they are executed directly by the database engine.

This post attempts to determine if the requirement to manually recompile native modules is any different for AG secondary replicas.

The results are interesting.

Disappearing Availability Groups

Cody Konior has a not-so-great magic trick:

I logged onto that node and the AG Dashboard looked okay at first glance. But the test was still failing when I re-ran it manually, so, I looked deeper.

I logged onto the second node and noticed the AG was completely gone. All the databases were in recovery but there was no sign of the AG at all. Nothing. Nada. Zip. (I don’t have any other words). It’s like it was never there.

At first I thought someone must have done something awful. I quickly poured a coffee while checking the default trace which usually records system-level configuration changes like dropping an entire replica but in this case nothing relevant showed up.

Read on for the answer, as well as action items to take if you’re actively using Availability Groups.

AG Secondary Stats Overwritten With Sample

Taiob Ali has run into an interesting issue:

Once I update my statistics with fullscan, with in 10~20 seconds some of the statistics on the same table are getting update on secondary with a sample pecent of rows. Meaning my best statistics are being overwritten with good (full vs sample) statistics. On primary node once I run “Update statistics Tablename with fullscan” . I see following about statistics status.

After 10~20 seconds of updating statistics in primary node if I check the status of the same on my secondary nodes, I see fullscan statistics is replaced by sample statistics. Look at the rows_sampled and last_updated column, you will see the sample row number and last_updated column time is within few seconds of update in primary. RowsModified column still showing zero records.

It’s happening on an Availability Group secondary.  Taiob has a workaround, so read on for that.

Clusterless Availability Groups

Allan Hirt talks about clusterless AGs:

A quick history lesson: through SQL Server 2016, we have three main variants of AGs:

  • “Regular” AGs (i.e. the ones deployed using an underlying Windows Server failover cluster [WSFC] requiring Active Directory [AD]; SQL Server 2012+)
  •  AGs that can be deployed without AD, but using a WSFC and certificates (SQL Server 2016+ with Windows Server 2016+)
  • Distributed AGs (SQL Server 2016+)

SQL Server v.Next (download the bits here) adds another variant which is, to a degree, a side effect of how things can be deployed in Linux: AGs with no underlying cluster. In the case of a Windows Server-based install, this means that there could be no WSFC, and for Linux, currently no Pacemaker.

Read on for more details, including limitations and expectations.

Thinking About Availability Group Outages

Brent Ozar reminds us to think about graceful degradation of applications:

There’s a gray bar across the top that says, “This site is currently in read-only mode; we’ll return with full functionality soon.”

That’s not a hidden feature of Always On Availability Groups. Rather, it’s a hidden feature of really dedicated developers whose application:

This is where a bit of foresight and hard work can really pay off.  Read the whole thing.

Network Latency And Availability Groups

Anthony Nocentino discusses network latency and how that can affect Availability Groups:

Now let’s talk about Availability Group replication and network latency. Availability Groups replicate data over your network using Database Mirroring Endpoints which are TCP sockets used to move data between the primary and it’s replicas. When designing Availability Groups, we often think about things in terms of bandwidth…how much data do I need to move between my replicas. But there’s another design factor your need to consider, network latency. Why? Hint, it’s not going to have anything to do with synchronous availability mode and HADR_SYNC_COMMIT waits. Let’s talk about some fundamentals of TCP for a second.

Click through for some discussion of TCP fundamentals.

Testing Transactional Replication

Jes Borland wraps up her series on transactional replication from an on-prem Availability Group to Azure SQL Database:

Congratulations, you’ve configured a remote distributor, configured all of your AG replicas as publishers, and configured your SQL Database as a subscriber! Now you want to ensure that transactions are replicating to the database, and that they continue to do so if there is a failover in the AG.

Read on for the two testing scenarios.

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