Elastic cluster

You can scale your cluster up or down to meet the needs of your database. The most common case is to add nodes to your database cluster to accommodate more data and provide better query performance. However, you can scale down your cluster if you find that it is overprovisioned or if you need to divert hardware for other uses.

You scale your cluster by adding or removing nodes. Nodes can be added or removed without having to shut down or restart the database. After adding a node or before removing a node, Vertica begins a rebalancing process that moves data around the cluster to populate the new nodes or move data off of nodes about to be removed from the database. During this process, data can be exchanged between nodes that are not being added or removed to maintain robust intelligent K-safety. If Vertica determines that the data cannot be rebalanced in a single iteration due to a lack of disk space, then the rebalance is done in multiple iterations.

To help make data rebalancing due to cluster scaling more efficient, Vertica locally segments data storage on each node so it can be easily moved to other nodes in the cluster. When a new node is added to the cluster, existing nodes in the cluster give up some of their data segments to populate the new node and exchange segments to keep the number of nodes that any one node depends upon to a minimum. This strategy keeps to a minimum the number of nodes that may become critical when a node fails (see Critical Nodes/K-safety in an Enterprise Mode database). When a node is being removed from the cluster, all of its storage containers are moved to other nodes in the cluster (which also relocates data segments to minimize nodes that may become critical when a node fails). This method of breaking data into portable segments is referred to as elastic cluster, since it makes enlarging or shrinking the cluster easier.

The alternative to elastic cluster is to resegment all of the data in the projection and redistribute it to all of the nodes in the database evenly any time a node is added or removed. This method requires more processing and more disk space, since it requires all of the data in all projections to essentially be dumped and reloaded.

Elastic cluster scaling factor

In a new installation, each node has a scaling factor that specifies the number of local segments (see Scaling factor). Rebalance efficiently redistributes data by relocating local segments provided that, after nodes are added or removed, there are sufficient local segments in the cluster to redistribute the data evenly (determined by MAXIMUM_SKEW_PERCENT). For example, if the scaling factor = 8, and there are initially 5 nodes, then there are a total of 40 local segments cluster wide.

If you add two additional nodes (7 nodes) Vertica relocates 5 local segments on 2 nodes, and 6 such segments on 5 nodes, resulting in roughly a 16.7% skew. Rebalance chooses relocates local segments only if the resulting skew is less than the allowed threshold, as determined by MAXIMUM_SKEW_PERCENT. Otherwise, segmentation space (and hence data, if uniformly distributed over this space) is evenly distributed among the 7 nodes, and new local segment boundaries are drawn for each node, such that each node again has 8 local segments.

Enabling elastic cluster

You enable elastic cluster with ENABLE_ELASTIC_CLUSTER. Query the ELASTIC_CLUSTER system table to verify that elastic cluster is enabled:

=> SELECT is_enabled FROM ELASTIC_CLUSTER;
 is_enabled
------------
 t
(1 row)