In SQL Server Always On Availability Groups (AG), quorum is essential to maintain high availability and ensure the correct functioning of the cluster. Quorum determines how many voting nodes must be online and in agreement for the cluster to operate. Understanding quorum types helps in configuring and managing Always On AG effectively.
Types of Quorum in
SQL Server Always On AG:
- Node Majority:
- Definition: Quorum is maintained by a majority of the nodes in
the cluster. Suitable for clusters with an odd number of nodes.
- Usage: Common in
multi-node clusters where having a majority is easier to achieve.
- Example: A 3-node
cluster requires at least 2 nodes to be online for quorum.
- Node and Disk Majority:
- Definition: A majority is formed using both the nodes and a disk
witness. The disk witness acts as an additional vote.
- Usage: Useful for
clusters with an even number of nodes to avoid split-brain scenarios.
- Example: A 4-node
cluster with a disk witness requires 2 nodes and the disk witness to
maintain quorum.
- Node and File Share
Majority:
- Definition: Similar to Node and Disk Majority, but uses a file
share witness instead of a disk. The file share witness provides an
additional vote.
- Usage: Preferred in
virtualized or cloud environments where a shared disk is not feasible.
- Example: A 4-node
cluster with a file share witness requires 2 nodes and the file share
witness to maintain quorum.
- No Majority (Disk Only):
- Definition: Only the disk witness has a vote, and quorum is
maintained as long as the disk witness is online.
- Usage: Rarely used
in Always On AG, as it creates a single point of failure. Typically seen
in traditional failover clusters.
- Example: A 2-node
cluster using only a disk witness, where the disk witness decides quorum.
- Node and
Cloud Witness Majority:
- Definition: Combines the voting power of
the cluster nodes with a cloud-based witness stored in Microsoft Azure
Blob Storage. The cloud witness acts as an additional vote in the
cluster.
- Usage: Ideal for geographically
distributed clusters, hybrid environments, or clusters without reliable
access to a shared disk or file share.
- Example: In a 4-node cluster, 2 nodes
and the cloud witness must be online to maintain quorum.
Benefits of Cloud Witness:
- No
additional hardware requirements:
Unlike a disk witness, which needs shared storage, or a file share
witness, which needs a reliable file server, Cloud Witness only requires
an internet connection and an Azure Blob Storage account.
- Resilience
in multi-site clusters:
Cloud Witness is perfect for multi-site or distributed clusters, as it
remains independent of local infrastructure, adding a layer of high
availability.
- Simplified
management: Azure's
integration with the Windows Failover Cluster Manager simplifies the configuration
and management of the witness.
When to Use Cloud Witness:
- Cloud or
hybrid environments: When
running SQL Server Always On AG in cloud or hybrid environments, Cloud
Witness provides a simple, scalable solution.
- No shared
storage: In
scenarios where traditional shared storage for disk witness or file share
witness is impractical.
- Geo-distributed
clusters: Cloud
Witness is ideal for clusters spanning multiple geographical regions where
local file share or disk-based solutions aren't feasible.
Choosing the Right
Quorum:
- Odd number of nodes: Use Node
Majority for simplicity and effective fault tolerance.
- Even number of nodes: Use Node
and Disk Majority or Node and File Share Majority to add a
tie-breaking vote and avoid split-brain scenarios.
- Virtual/Cloud Environments: Node and File Share Majority is often preferred
as it avoids the complexity of disk dependencies.
By adding Cloud Witness to the list of
quorum types, SQL Server Always On AG gains more flexibility and fault
tolerance, especially in hybrid or cloud-native architectures.
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