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Category: HA / DR

Split-Brain Scenarios in PostgreSQL Clusters

Published 2025-06-02 by Kevin Feasel

Semab Tariq knows that an application cannot serve two masters:

In this blog post, we will try to explore a critical failure condition known as a split-brain scenario that can occur in PostgreSQL HA clusters. We will first see what split-brain means, and then how it can impact PostgreSQL clusters, and finally discuss how to prevent it through architectural choices and tools available in the PostgreSQL ecosystem

Click through for an explanation of split-brain and what can cause this problem. Additionally, Semab includes several tips on how to limit the likelihood of a split-brain scenario occurring.

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HA/DR in Oracle with Data Guard

Published 2025-05-30 by Kevin Feasel

Kellyn Gorman takes a peek at Oracle Data Guard:

In its traditional, (and free) configuration, Oracle Data Guard operates in an active/passive architecture.  This incredibly well-designed and valuable solution from Oracle which comes included with the Enterprise Edition has as part of its architecture:

  • A primary database, which is an active, accessible database system.
  • One or more standby databases, which are passive replicas that continuously receive redo data from the primary.

Click through for an overview of the product.

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Database Snapshots in High-Availability Setups

Published 2025-05-14 by Kevin Feasel

Stephen Planck adds one more layer of complexity:

SQL Server’s database-snapshot feature is a wonderfully simple tool: at the instant you create the snapshot, every page in the database is marked “copy-on-write.” Nothing is copied across the wire, no blocking locks appear, and the snapshot opens immediately as a read-only database on the local replica. Queries against the snapshot see the world exactly as it looked at that moment while the live workload keeps changing pages in the primary data files. Because snapshots live only in sparse files on the server that owns them, they are not a replacement for backups—but they are perfect for ad-hoc reporting, quick “before-and-after” comparisons, or a safety net when you want an easy way to back out a risky change that should finish within minutes or hours.

But read on to see how they interact with high-availability features such as transactional replication and availability groups.

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Failover Groups in Azure SQL Database

Published 2025-02-21 by Kevin Feasel

Mika Sutinen looks at some interesting functionality:

One of the interesting features in Azure SQL Database is the Failover Groups. It allows you to manage replication of an Azure SQL database, or group of databases, to another logical server. The reason I’ve bolded the manage replication is, that the replication itself is handled by active geo-replication, which is also a feature of Azure SQL Database.

Read on to see how these are different and why you might want to use failover groups.

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Using Kubernetes with Distributed Availability Groups

Published 2024-11-19 by Kevin Feasel

Andrew Pruski has a guide for us:

A while back I wrote about how to use a Cross Platform (or Clusterless) Availability Group to seed a database from a Windows SQL instance into a pod in Kubernetes.

I was talking with a colleague last week and they asked, “What if the existing Windows instance is already in an Availability Group?”

This is a fair question, as it’s fairly rare (in my experience) to run a standalone SQL instance in production…most instances are in some form of HA setup, be it a Failover Cluster Instance or an Availability Group.

Read on for the tutorial. There are quite a few steps involved.

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Online DR from SQL Server 2022 and Azure SQL MI Now Available

Published 2024-10-10 by Kevin Feasel

Djordje Jeremic announces general availability of one of the key selling points from SQL Server 2022:

Today, we are announcing the general availability of the following two major capabilities of the Managed Instance link feature with SQL Server 2022:

  • Two-way failover between SQL Server 2022 and SQL Managed Instance through the link to unlock true disaster recovery (DR) with Azure
  • Creating a link from SQL Managed Instance to SQL Server 2022 to unlock off-PaaS data mobility for regulatory and dev/test scenarios 

Click through for more detail.

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pgBackRest and Standby Server Backups

Published 2024-09-23 by Kevin Feasel

Stefan Fercot does some explaining:

Recently, we’ve received many questions about how to take backups from a standby server using pgBackRest. In this post, I’d like to clarify one of the most frequently asked questions and address a common misconception for new users.

First of all, it’s important to understand that taking a backup exclusively from the standby server is not currently possible. When you trigger a backup from the standby, pgBackRest creates a standby backup that is identical to a backup performed on the primary. It does this by starting/stopping the backup on the primary, copying only files that are replicated from the standby, then copying the remaining few files from the primary.

Read on to learn more and to see an example of how this works.

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Resuming Data Movement for an Availability Group

Published 2024-08-21 by Kevin Feasel

Chad Callihan gets things moving after a few 1s without enough 0s clog up the pipe:

Keeping an Always On Availability Group healthy is crucial, and seeing a non-synchronizing database in an Always On High Availability Group can give you a sinking feeling (pardon the pun). Disregarding the reason for the syncing issue, there are a few ways to resume syncing and get your setup back in the green.

Let’s look at resuming using the SSMS GUI and running a SQL statement.

Read on for the process. I appreciate that Chad also includes the T-SQL operation to do this.

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SQL Server on Linux for Disaster Recovery

Published 2024-08-19 by Kevin Feasel

Mika Sutinen shares a use case:

SQL Server on Linux has been an option for quite a while already, but I have to admit that personally, I have previously not been a very excited about that idea. This is mostly due to some complexities and limitations of SQL Server on Linux. However, the recent CrowdStrike incident made it quite obvious, that there’s a great use case for it. And that use case is Disaster Recovery.

The idea of having some operating system variety makes sense, though read the whole thing, as some companies will use components that SQL Server doesn’t offer for Linux.

And if you want a deep dive into using SQL Server on Linux, I’ll do a bit of self-promotion and reference my video series on the topic.

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