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Category: Availability Groups

A Review of Distributed Availability Groups

Joey D’Antoni shares some thoughts on distributed Availability Groups in SQL Server 2016 and later:

I’m writing this post because I’ve been mired in configuring a bunch of distributed availability groups for a client, and while the feature is technically solid, the lack of tooling can make it a challenge to implement. Specifically, I’m implementing these distributed AGs (please don’t use the term DAG as you’ll piss off Allan Hirt, but more importantly its used in Microsoft Exchange High Availability, so it’s taken) in Azure which adds a couple of additional changes because of the need for load balancers. You should note this feature is Enterprise Edition only, and is only available starting with SQL Server 2016.

Read on for some of the positives around distributed AGs, as well as some negatives (mostly around the lack of tooling).

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Restoring a Database in an Availability Group

Rajendra Gupta walks us through the process of restoring a database which is currently in an Availability Group:

You might think a question here– We can take production database backup and restore it on the development database. What difference does it make in a standalone database restore or availability group database restore?

Database restore works with the standalone database, but if the database is configured in the availability group, we cannot directly restore the database. It requires additional steps because of the AG configurations. Our database should be in the same state (AG synchronized) after the database restores as well.

In this article, let’s cover the steps to restore an existing availability group database in the SQL Server Always On Availability Group.

Read on for the answer.

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Working with Read-Only Endpoints in Azure SQL Database

Arun Sirpal takes us through one method for improving performance in Azure SQL Database:

One of the main benefits of configuring active geo-replication for Azure SQL Database is leveraging the read-only endpoint, a good technique to split away read only activity from OLTP based workloads. This means that there is no reason why you cannot point users to these databases via tools such as Power BI as highlighted below.

But there are some things to keep in mind, as Arun points out.

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Fixing Availability Group Issues with Alerts

Wayne Sheffield automates a few problems away:

The first of the Availability Group issues to discuss is that, for whatever reason, data is no longer moving between the primary replica and a secondary replica. This puts the Data Movement in a Suspended state.

If the data movement remains suspended for too long, you might have to take some undesired actions to get things back in sync. Things like removing the database from the AG, restoring log files, then reattaching it to the AG. When the data movement becomes suspended, we want to get it flowing again as soon as possible. Let’s have SQL Server try to get the data flowing again.

Read on for more, including a second issues that Wayne helps solve.

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Lessons Learned from Migrating to SQL Server 2017 with Availability Groups

Lee Markum has an after-action report:

In late 2019, a project that had been in progress for well over a year finally came to a conclusion.  I had collaborated with a number of people to migrate a stand alone SQL Server 2008 and two stand alone SQL Server 2008 R2 instances.  Each stand alone SQL Server was migrated to a three node Availability Group. Here are a few things learned along the way.

Click through for some good advice.

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A Note on Distributed Network Names

Allan Hirt provides an explanation around Distributed Network Names when building Windows Server Failover Clusters on Windows Server 2019:

The new Windows Server 2019 DNN functionality does have a side effect that does affect Azure-based configurations. When creating a WSFC, Windows Server 2019 detects that the VM is running in Azure and will use a DNN for the WSFC name. This is the default behavior.

I clipped this paragraph specifically because Allan uses both “affect” and “effect” correctly, and I wanted to call that out. Do read the rest of it as well.

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Availability Group Bug when Removing and Adding the Same Database

Josh Darnell takes us through a tricky problem:

I came across a bug in SQL Server 2016 where the Availability Group (AG) health check can get stuck in an infinite loop after removing and re-adding a database from an AG.

Unfortunately, I don’t know exactly what version this bug was introduced. I first noticed the problem on SQL Server 2016 SP2 CU7 GDR (13.0.5366.0). It may have existed before then, but I never encountered it.

Read on for a workaround. And hopefully there will be a proper fix soon. Also, it’d be interesting to see if it can be reproduced in 2017 or 2019.

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Frequently Asked Availability Group Questions

Sean Gallardy answers the voices swarming in his head:

Q3: Do my secondary replicas really need to be the same level of hardware as my primary replicas?
Sean: They should, this assumes you are eventually going to fail over to them or they might need to run your workload. It does *not* assume they will be used for readable workloads. Remember that the secondary replica is already accepting and writing log data, redoing that data, and then if you’re running a bunch of read workloads on top of that it might actually be *busier* than your primary. Yes, I’ve witnessed secondary replicas getting hammered harder than their associated primary.

Read on for informative and entertaining answers, and avoid those rusty nails.

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