Planning a Large Cluster

There are two factors you should consider when planning to expand your database cluster to the point that it needs to use large cluster:

• How many control nodes should your database cluster have?
• How should those control nodes be distributed?

Determining the Number of Control Nodes

When you manually enable large cluster or add enough nodes to trigger Vertica to enable it automatically, a subset of the cluster nodes become control nodes. In subclusters with fewer than 16 nodes, all nodes are control nodes. In many cases, you can set the number of control nodes to the square root of the total number of nodes in the entire Enterprise Mode cluster, or in Eon Mode subclusters with more than 16 nodes. However, this formula for calculating the number of control is not guaranteed to always meet your requirements.

When choosing the number of control nodes in a database cluster, you must balance two competing considerations:

• If a control node fails or is shut down, all nodes that depend on it are cut off from the database. They are also down until the control node rejoins the database. You can reduce the impact of a control node failure by increasing the number of control nodes in your cluster.
• The more control nodes in your cluster, the greater the load on the spread service. In cloud environments, increased complexity of the network environment broadcast can contribute to high latency. This latency can cause messages sent over the spread service to take longer to reach all of the nodes in the cluster.

In a cloud environment, experience has shown that 16 control nodes balances the needs of reliability and performance. In an Eon Mode database, you must have at least one control node per subcluster. Therefore, if you have more than 16 subclusters, you must have more than 16 control nodes.

In an Eon Mode database, whether on-premises or in the cloud, consider adding more control nodes to your primary subclusters than to secondary subclusters. Only nodes in primary subclusters are responsible for maintaining K-Safety in an Eon Mode database. Therefore, a control node failure in a primary subcluster can have greater impact on your database than a control node failure in a secondary subcluster.

In an on-premises Enterprise Mode database, consider the physical layout of the hosts running your database when choosing the number of control nodes. If your hosts are spread across multiple server racks, you want to have enough control nodes to distribute them across the racks. Distributing the control nodes helps ensure reliability in the case of a failure that involves the entire rack (such as a power supply or network switch failure). You can configure your database so no node depends on a control node that is in a separate rack. Limiting dependency to within a rack prevents a failure that affects an entire rack from causing additional node loss outside the rack due to control node loss.

Selecting the number of control nodes based on the physical layout also lets you reduce network traffic across switches. By having dependent nodes on the same racks as their control nodes, the communications between them remain in the rack, rather that traversing a network switch.

You might need to increase the number of control nodes to evenly distribute them across your racks. For example, on-premises Enterprise Mode database has 64 total nodes, spread across three racks. The square root of the number of nodes yields 8 control nodes for this cluster. However, you cannot evenly distribute eight control nodes among the three racks. Instead, you can have 9 control nodes and evenly distribute three control nodes per rack.

Influencing Control Node Placement

After you determine the number of nodes for your cluster, you need to determine how to distribute them among the cluster nodes. Vertica chooses which nodes become control nodes. You can influence how Vertica chooses the control nodes and which nodes become their dependents. The exact process you use depends on your database's mode:

• Enterprise Mode on-premises database: Define fault groups to influence control node placement. Dependent nodes are always in the same fault group as their control node. You usually define fault groups that reflect the physical layout of the hosts running your database. For example, you usually define one or more fault groups for the nodes in a single rack of servers. When the fault groups reflect your physical layout, Vertica places control nodes and dependents in a way that can limit the impact of rack failures. See Fault Groups for more information.
• Eon Mode database: Use subclusters to control the placement of control nodes. Each subcluster must have at least one control node. Dependent nodes are always in the same subcluster as their control nodes. You can set the number of control nodes for each subcluster. Doing so lets you assign more control nodes to primary subclusters, where it's important to minimize the impact of a control node failure.

How Vertica Chooses a Default Number of Control Nodes

Vertica can automatically choose the number of control nodes in the entire cluster (when in Enterprise Mode) or for a subcluster (when in Eon Mode). It sets a default value in these circumstances:

• When you pass the default keyword to the --large-cluster option of the install_vertica script (see Enable Large Cluster When Installing Vertica).
• Vertica automatically enables large cluster when your database cluster grows to 120 or more nodes.
• Vertica automatically enables large cluster for an Eon Mode subcluster if you create it with more than 16 nodes. Note that Vertica does not enable large cluster on a subcluster you expand past the 16 node limit. It only enables large clusters that start out larger than 16 nodes.

The number of control nodes Vertica chooses depends on what triggered Vertica to set the value.

If you pass the --large-cluster default option to the install_vertica script, Vertica sets the number of control nodes to the square root of the number of nodes in the initial cluster.

If your database cluster reaches 120 nodes, Vertica enables large cluster by making any newly-added nodes into dependents. The default value for the limit on the number of control nodes is 120. When you reach this limit, any newly-added nodes are added as dependents. For example, suppose you have a 115 node Enterprise Mode database cluster where you have not manually enabled large cluster. If you add 10 nodes to this cluster, Vertica adds 5 of the nodes as control nodes (bringing you up to the 120-node limit) and the other 5 nodes as dependents.

In an Eon Mode database, each subcluster has its own setting for the number of control nodes. Vertica only automatically sets the number of control nodes when you create a subcluster with more than 16 nodes initially. When this occurs, Vertica sets the number of control nodes for the subcluster to the square root of the number of nodes in the subcluster.

For example, suppose you add a new subcluster with 25 nodes in it. This subcluster starts with more than the 16 node limit, so Vertica sets the number of control nodes for subcluster to 5 (which is the square root of 25). Five of the nodes are added as control nodes, and the remaining 20 are added as dependents of those five nodes.

Even though each subcluster has its own setting for the number of control nodes, an Eon Mode database cluster still has the 120 node limit on the total number of control nodes that it can have.