To achieve optimal performance in SQL Server AlwaysOn Availability Groups, especially for a rapid failover in a multi-subnet environment, it’s recommended to adjust the “RegisterAllProvidersIP“, “HostRecordTTL“, and “MultiSubnetFailover” settings as follows:
If driver supports MultiSubnetFailover in Connection Strings:
MultiSubnetFailover = true
RegisterAllProvidersIP = 1
HostRecordTTL = 60
If MultiSubnetFailover can be used in the connection strings, then the recommended settings would be:
- RegisterAllProvidersIP: This should be set to 1. This is because when RegisterAllProvidersIP is set to 1, it will reduce the re-connection time after a failover for clients whose client connection strings specify MultiSubnetFailover = True. Note that to take advantage of the listener multi-subnet feature, your clients might require a data provider that supports the MultiSubnetFailover keyword.
- HostRecordTTL: The document does not specify an explicit value for HostRecordTTL when MultiSubnetFailover can be used in the connection strings. By default, clients cache cluster DNS records for 20 minutes, but depending on your network configuration and load, this value may need adjustment. Reducing this value could help clients reconnect more quickly after a failover, but it might also increase the traffic to the DNS servers due to more frequent DNS record refreshes. Therefore, it’s advisable to monitor and adjust this setting according to the specific needs and behaviors of your environment.
Keep in mind that using these settings might also require updating or reconfiguring your client software or drivers to fully support the MultiSubnetFailover feature.
If driver DOES NOT support MultiSubnetFailover in Connection Strings:
MultiSubnetFailover = false
RegisterAllProvidersIP = 0
HostRecordTTL = 60
If MultiSubnetFailover cannot be used in the connection strings, then the recommended settings would be:
- RegisterAllProvidersIP: This should be set to 0. When RegisterAllProvidersIP = 1, any clients whose connection strings do not use MultiSubnetFailover = True, will experience high latency connections because these clients attempt connections to all IPs sequentially. If RegisterAllProvidersIP is changed to 0, the active IP address is registered in the Client Access Point in the WSFC cluster, reducing latency for legacy clients.
- HostRecordTTL: The document does not specify an explicit value for HostRecordTTL when MultiSubnetFailover cannot be used in the connection strings. However, it suggests that by reducing the HostRecordTTL, the Time to Live (TTL) for the cached record, legacy clients may reconnect more quickly. But keep in mind that reducing the HostRecordTTL setting may result in increased traffic to the DNS servers due to more frequent DNS record refreshes.
As always, these settings should be adjusted based on your particular needs and the behavior of your network environment.
Reasoning for those two options for RegisterAllProvidersIP and HostRecordTTL:
The choice of these two settings, RegisterAllProvidersIP and HostRecordTTL, is primarily driven by the need to optimize connection times and reduce latency in the context of SQL Server’s Always On Availability Groups and Always On Failover Cluster Instances. Each of these settings plays a unique role in determining how client connections behave during a failover event.
- RegisterAllProvidersIP: The purpose of this setting is to control how IP addresses from the Failover Cluster are registered with the DNS (Domain Name System).
- When RegisterAllProvidersIP is set to 1, it implies that all the IP addresses associated with the Failover Cluster Instance are registered in DNS. This is beneficial when MultiSubnetFailover is set to True in the client’s connection string, because it allows the client to attempt connections to all registered IP addresses in parallel, thus speeding up the re-connection time after a failover.
- However, if MultiSubnetFailover is not set or not supported (i.e., set to False), the clients will attempt connections to all IPs sequentially. This could lead to high latency connections as the clients would try to connect to potentially non-responsive or wrong subnet IPs before eventually connecting to the correct active IP. In this case, setting RegisterAllProvidersIP to 0 would be more efficient, as only the active IP address would be registered with the DNS, leading to faster client connections.
- HostRecordTTL (Time to Live): This setting determines how long the DNS records are cached by clients and other DNS servers.
- When MultiSubnetFailover is supported and used, the default TTL setting is generally sufficient as clients can quickly reconnect using all available IPs.
- However, when MultiSubnetFailover is not used or supported, reducing the TTL value may benefit clients as they can update their cache more frequently and thus reconnect more quickly after a failover. This comes at the cost of increased DNS traffic due to more frequent cache updates. Therefore, it’s a trade-off between faster client re-connection times and potential increased network load.
These settings help balance the need for quick client recovery during failover scenarios against the constraints of the specific network environment and client capabilities. They provide options for administrators to tailor the system behavior according to their specific use case and needs.
What is this MultiSubnetFailover Feature?
MultiSubnetFailover is a feature introduced in SQL Server 2012 designed to facilitate a faster failover with Always On Availability Groups and Always On Failover Cluster Instances.
The concept of failover is central to high availability and disaster recovery strategies. In a database environment, failover is the process by which a system automatically transfers control to a redundant or standby system upon detecting a failure or anomaly in the primary system.
The MultiSubnetFailover feature is particularly useful in a multi-subnet environment, where Always On Availability Groups and Always On Failover Cluster Instances are spread across different subnets, often in geographically dispersed locations. Such configurations are typical in disaster recovery strategies where replicas of the primary system are maintained in a different location to protect against site-wide failures.
When the MultiSubnetFailover is set to True in the connection string:
1. It enables SQL Server to swiftly failover to a different subnet when required, making the system more resilient and reducing downtime.
2. It can significantly enhance the speed of the failover process by enabling more aggressive retries during the connection process, which results in quicker failover times.
3. In the context of named instance resolution, it adds support for Always On Failover Cluster Instances with multiple subnet endpoints, which can streamline connectivity in complex network configurations.
Therefore, if your application’s connection strings support the MultiSubnetFailover property and your environment is set up with multi-subnet Always On Availability Groups or Always On Failover Cluster Instances, using this feature can substantially improve your failover performance and overall database availability.
However, it’s important to note that not all clients or database drivers support the MultiSubnetFailover property, so compatibility should be confirmed before implementation.