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Category: Availability Groups

Plan Forcing and Availability Groups

Published 2019-11-01 by Kevin Feasel

Milos Radivojevic shows an example where plan forcing might not work:

In my previous post about Query Store, you saw that persistence of forced plans does not imply that they are used. This post brings another example where forced plans are still in the system, with no failures, but they are not applied against a desired query.

For this demo, we need a database which is a part of the availability group. 

Click through to see why failover might cause your forced queries no longer to work.

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MultiSubNetFailover and Availablity Groups

Published 2019-11-01 by Kevin Feasel

William Assaf strongly advises setting MultiSubNetFailover=True in connection strings when using Availability Groups:

I received an email from a client who is having issues with third-party applications connecting to their three-subnet SQL Server Availability Group. After an exchange with Microsoft Support, they discovered that the applications weren’t specifying MultiSubNetFailover = True in their connection strings. As a result, because RegisterAllProvidersIP = 1 in the cluster, connections were randomly experiencing high latency upon connecting, as client-side DNS queries over time had a 66% chance of returning the wrong IP from the listener.

They set RegisterAllProvidersIP = 0, but before you take that as advice keep reading.

Read on to learn why this is not a first-best solution.

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Building a SQL Cluster Lab

Published 2019-10-31 by Kevin Feasel

Ryan Adams has started a series on building a Windows cluster in Hyper-V and layering SQL Server on top of it:

Before we start to build a SQL Cluster Lab, let’s look at the desired result. You will build a 3-node cluster replicating an environment that has two data centers. As a result, the first two nodes will reside in data center 1 and the third node in data center 2. We are creating this architecture because it is the most common architecture I see for Availability Groups. It is multi-subnet and can solve for both HA and DR.

You will notice the domain controller in the middle. That piece is certainly not representative of a production environment. However, we are using it in our lab for several different functions and being a router is one of them.

Part 1 is the only part which is currently up, but this looks like it will be a good one. Go buy a couple more sticks of RAM for your PC and get reading.

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Making SQL Agent Jobs AG-Aware

Published 2019-10-22 by Kevin Feasel

Stuart Moore shows how you can use dbatools to make SQL Agent jobs Availability Group-aware:

What do I mean by Availability Group aware? When running on an Availability Group, one SQL Server instance ‘owns’ the database at any point in time, but the SQL Agent jobs have to be replicated across all of the instances in the cluster. So you want to make sure that your SQL Server Agent jobs only do work on the instance that currently owns the Availability Group.

Doing this is pretty simple. Below is a piece of T-SQL that checks if the current SQL Server Instance is the primary instance in the AG. If it isn’t then we exit with an error.

Read on to see how, and how you can use dbatools to automate this work.

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Upgrading Servers in an Availability Group

Published 2019-10-18 by Kevin Feasel

Thomas Rushton has a checklist for upgrading servers connected by an Availability Group:

This is a checklist put together and followed for an upgrade of a pair of physical SQL Server 2012 servers which hosted a single availability group of several terabytes of data with minimal downtime.

The availability group was configured with synchronous commit and automatic failover.

Minimizing downtime here is great, but it’s not automatic: you still need to do work on your end to get this right.

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AGs and Redo Latency

Published 2019-10-18 by Kevin Feasel

David Fowler explains that even in synchronous mode, there can still be redo latency for Availability Groups:

There’s a misconception that if you have a synchronous replica, that replica has to commit any transactions that are passed to it by the primary before the primary can then commit those same transactions. This ensures that there is no possibility of data loss and that the data would always be available on the secondary.

With this misconception comes our problem.

Read on to understand the full issue.

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MSDTC and Availability Groups

Published 2019-10-16 by Kevin Feasel

Ryan Adams provides guidance on using distributed transactions against Availability Groups:

A paramount concept to understand is how to make the DTC highly available.  We can see from the precedence order that SQL Server will use the local DTC out of the box.  This makes it appear that everything is working, and it is, but it is not exactly highly available.

I see a lot of customers leave it configured this way because they either don’t know the ramifications or do not realize they are using the MSDTC (Linked Servers). Since it simply works out of the box, things get left this way until they end up with a suspect database and error messages that look like this:

“SQL Server detected a DTC/KTM in-doubt transaction with UOW  {598B7EDD-F7A1-9DC1-8D3E-303A4C93AAB4}.Please resolve it following the guideline for Troubleshooting DTC Transactions.”

Read the whole thing. There are a lot of small areas between processes where things can fail, and the combination of DTC + AGs is no different.

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Rolling Windows Upgrades with AGs + WSFC

Published 2019-09-03 by Kevin Feasel

Allan Hirt shows how you can combine Availability Groups with Windows Server Failover Clusters and upgrade the operating system version while keeping your SQL Servers running:

The configuration for a cluster rolling upgrade allows for mixed Windows Server versions to coexist in the same WSFC. This is NOT a deployment method. It is an upgrade method. DO NOT use this for normal use. Unfortunately, Microsoft did not put a time limit on how long you can run in this condition, so you could be stupid and do something like have a mixed Windows Server 2012 R2/2016 WSFC. Fire, ready, aim. The WSFC knows about this and you’ll see a warning with an Event ID of 1548.

Read on for a summary of what Allan has learned in doing this.

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