High availability on Rudi Bruchez

SQL Server Standard: understanding Basic Availability Groups (BAG)

Tue, 27 Jan 2026 00:00:00 +0000

TL;DR

Basic Availability Groups (BAG) give SQL Server Standard edition users high availability similar to the Always On AG of the Enterprise edition, but with significant limitations:

One group per instance.
One database per group.
No reads on the secondary replica.
No backups on the secondary.
Two nodes maximum, primary and secondary.

For a long time, using Always On Availability Groups required an Enterprise licence, leaving Standard edition users with Database Mirroring. Since SQL Server 2016, Microsoft has offered Basic Availability Groups (BAG) as the replacement for mirroring in the Standard edition.

Always On: mastering the timeouts

Tue, 27 Jan 2026 00:00:00 +0000

TL;DR

To avoid spurious failovers in an Always On environment, adjust these settings:

Lease Timeout: raise it if your server goes through CPU load spikes.
HealthCheck Timeout: raise it if your server is slow to answer diagnostic requests.

When you run AlwaysOn Availability Groups, it is not unusual to get spurious failovers while your system is working perfectly well.

It tends to happen at night, and more rarely during the day. The most common cause is a temporary unavailability of the system, which makes the cluster health checks time out. A typical example is a VM freeze while backup software takes a snapshot.

Troubleshooting AlwaysOn availability groups

Mon, 01 Jan 0001 00:00:00 +0000

Diagnosing the availability groups

Collect the SQL Server error logs from the SQL Server log directory.
Collect the files of the AlwaysOn_health event session: the alwayson_health*.xel files, in the same SQL Server log directory.

You can find the SQL Server log directory with this query:

SELECT
	LEFT(CAST(SERVERPROPERTY('ErrorLogFileName') as nvarchar(max)),
		LEN(CAST(SERVERPROPERTY('ErrorLogFileName') as nvarchar(max))) - LEN('ERRORLOG'));

Diagnosing WSFC

You can export the cluster logs with the Get-ClusterLog cmdlet:

Import-Module FailoverClusters
Get-ClusterLog -UseLocalTime -Destination C:\temp\

Microsoft also provides a tool that collects the cluster diagnostics and logs. You can install it by following these instructions.

Use dbatools for AlwaysOn availability groups

Mon, 01 Jan 0001 00:00:00 +0000

List the groups and the replica states

Set-DbatoolsInsecureConnection
Get-DbaAgReplica -SqlInstance localhost | ft Name, AvailabilityGroup, FailoverMode, Role, RollupSynchronizationState

Fail all availability groups over to a specific node

Set-DbatoolsInsecureConnection

Get-DbaAgReplica -SqlInstance localhost | where {$_.Role -eq "Secondary" -and $_.RollupSynchronizationState -eq "Synchronized"} | ft Name, AvailabilityGroup, FailoverMode, Role, RollupSynchronizationState

Get-DbaAgReplica -SqlInstance localhost
Get-DbaAgReplica -SqlInstance localhost | Switch-DbaAgReplica -TargetSqlInstance "SQLNODE2"

Get-DbaAgReplica -SqlInstance localhost | Move-DbaAgReplica -DestinationSqlInstance "SQLNODE2"

List the databases that are not synchronizing

Set-DbatoolsInsecureConnection
Get-DbaAgDatabaseReplicaState -SqlInstance localhost | where {$_.IsSuspended} | ft ReplicaServerName, AvailabilityGroup, DatabaseName, SuspendReason

Resume synchronization on the suspended databases

Set-DbatoolsInsecureConnection
Resume-DbaAgDbDataMovement -SqlInstance sql2 -AvailabilityGroup test

Get-DbaAgDatabase -SqlInstance sql2 | Resume-DbaAgDbDataMovement -Confirm:$false

Fail over every availability group

Set-DbatoolsInsecureConnection

$secondary = Get-DbaAgReplica -SqlInstance localhost | where {$_.Role -eq "Secondary" -and $_.RollupSynchronizationState -eq "Synchronized"} | select Name -First 1

Get-DbaAvailabilityGroup -SqlInstance $secondary.Name | Invoke-DbaAgFailover -Confirm:$false