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Category: HA / DR

Mixed MultiSubnetFailover Support on AGs

Andy Mallon continues a line of thought:

In yesterday’s post, I showed how to configure an availability group (AG) to use the RegisterAllProvidersIP=0 when you can’t get clients to connect using the MultiSubnetFailover=true connection string attribute.

I mentioned that you have to make some trade-offs when you set RegisterAllProvidersIP=0, and included this comparison:

But….when if you can eat your cake and have it, too?

In some cases, you’ll have some applications & clients that are not able to use MultiSubnetFailover=true, and other clients that can. Perhaps you’re working on updating a bunch of legacy Java apps to move from old jTDS drivers to the current Microsoft JDBC drivers that properly support MultiSubnetFailover=true. Parts of your codebase have been updated, and you want them to make use of the connection string attribute for fast cross-subnet failover. But other parts of your codebase are still being updated and rely on the RegisterAllProvidersIP cluster parameter to be false. Wouldn’t it be nice to have both?

Read on to learn how.

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So You Want to Fail Over a SQL Managed Instance

Danimir Ljepava takes us through user-initiated failover of SQL Managed Instances:

In August 2020, we have released a new feature user-initiated manual failover allowing to manually trigger a failover on SQL Managed Instance using PowerShell or CLI commands, or through invoking an API call.

Manually initiated failover on a managed instance will be an equivalent of the automated failover for high availability and software patches initiated automatically by the service. Manually invoking a failover on MI will help test end-to-end applications for fault resiliency on automatic failovers in case of planned or unplanned events before deploying to production. In addition to testing how failover impacts existing database sessions, it can also help verify if it changes the end-to-end performance due to changes in the network latency. In some cases if performance issues are encountered on SQL MI, manually invoking a failover to a new node can help mitigate the performance issue.

Read on to see how you can perform failover and how you can confirm that it worked.

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Azure SQL Database Business Continuity Options

James Serra covers business continuity scenarios with Azure SQL Database:

I have wrote a number of blogs on the topic of business continuity in SQL Database before (HA/DR for Azure SQL DatabaseAzure SQL Database high availabilityAzure SQL Database disaster recovery) but with a number of new features I felt it was time for a new blog on the subject, focusing on disaster recovery and not high availability.

Business continuity in Azure SQL Database and SQL Managed Instance refers to the mechanisms, policies, and procedures that enable your business to continue operating in the face of disruption, particularly to its computing infrastructure. In the most of the cases, SQL Database and SQL Managed Instance will handle the disruptive events that might happen in the cloud environment and keep your applications and business processes running.

James takes us through options available for Azure SQL Database as well as managed instances.

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An Overview of HADR Concepts with SQL Server

Kevin Hill walks through different topics around high availability and disaster recovery:

Replication—a very Special Snowflake:

SQL SERVER REPLICATION IS NEITHER HA NOR DR.

Not everything in a SQL Server user database CAN be replicated, such as users, or tables with no Primary Key. New objects are not automatically sent from Publisher to Subscriber. System databases are not replicated.

There’s plenty of good information in here, so check it out.

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Disaster Recovery for Your Workstation

Randolph West explains that disaster recovery isn’t just for your servers:

I just completed a chapter for another book where I spoke about the Recovery Point Objective (how much data you are prepared to lose) and Recovery Time Objective (how long you have to bring your environment up again) after a disaster, and while I never get tired of repeating myself, that’s SQL Server. What happens if your development environment — or workstation — experiences a catastrophic failure?

Or what if, say, you’re on a cruise ship in the middle of the ocean with Internet access and a phone (but no laptop) and your on-call person just died? (I’ll leave this as an exercise for the reader to decide if this really happened.)

The answer is, if we do a careful bit of planning using the same disaster recovery principles we already know, the impact could be minimal. Note that this post assumes that you have Internet access and are using Microsoft Windows as your environment.

Click through for some useful suggestions.

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Explaining the HA/DR Licensing Changes

Kevin Chant goes into the fairly recent licensing changes for SQL Server:

Which surprised me a bit because these licensing changes have been in-place for a while now. With this in mind, I thought I would discuss them here to raise awareness about the changes.

To clarify, in SQL Server 2019 there have been some big licensing changes about what you can and can’t do on a passive fail-over instance. Especially if you have Software Assurance.

Which I have to admit I am really excited about. Because it opens up some new possibilities which I will explain below. Of course, there are other significant updates in the licensing guide as well.

Read on for the details.

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High Availability Changes in SQL Server 2016

Martin Surasky looks at what SQL Server 2016 changed with respect to High Availability options:

AlwaysOn Availability Groups, first introduced in SQL Server 2012 is a feature that is conceptually similar to database mirroring. I’m going to assume you already know what AlwaysOn Availability Groups are in general, their main purpose and how they are different (essential aspects at least) from other technologies to provide replication such as Database Mirroring.

In SQL Server 2014, the significant enhancement to availability groups was the increase in the number of supported secondary replicas from three to eight. SQL Server 2016 includes a number of new enhancements

The biggest thing about it, as I recall, was stability: I wouldn’t have recommended too many places go into production with Availability Groups in 2012, but by 2016, many of the biggest bugs were ironed out.

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Creating a Failover Cluster Instance with Shared Storage

Ryan Adams wraps up a video series on setting up a SQL Server lab environment:

You are going to create a SQL Server Failover Cluster Instance in Part 4 of our series on how to build a SQL Cluster Lab. The FCI will only be installed on Node1 and Node2. FCIs require shared storage so you will make your domain controller an iSCSI target. Last you will create your FCI using the iSCSI drives you presented to the cluster. 

Click through for links to the entire series.

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High Availability Announcements from Microsoft

Allan Hirt looks at a couple announcements from Microsoft:

I’m going to discuss what I feel are the biggest game changers. I knew licensing was changing as I had conversations with Microsoft around this months ago. I was not sure what the final result was going to be, but I’m fairly pleased. Is it perfect? No, but it’s much better than it was.

You’ll definitely want to read Allan’s thoughts on Microsoft’s SQL Server licensing changes, as well as a private preview of Azure Shared Disks.

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MultiSubNetFailover and Availablity Groups

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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