RAM is allocated to Couchbase Server in two different configurable quantities, the Server Quota and Bucket Quota. For more information about creating and changing these two settings, see Section 7.4.2, “Other couchbase-cli Usage” and Section 6.3.1, “Creating and Editing Data Buckets”.
The Server Quota is the RAM that is allocated to the server when Couchbase Server is first installed. This sets the limit of RAM allocated by Couchbase for caching data for all buckets and is configured on a per-node basis. The Server Quota is initially configured in the first server in your cluster is configured, and the quota is identical on all nodes. For example, if you have 10 nodes and a 16GB Server Quota, ther is 160GB RAM available across the cluster. If you were to add two more nodes to the cluster, the new nodes would need 16GB of free RAM, and the aggregate RAM available in the cluster would be 192GB.
The Bucket Quota is the amount of RAM allocated to an individual bucket for caching data. Bucket quotas are configured on a per-node basis, and is allocated out of the RAM defined by the Server Quota. For example, if you create a new bucket with a Bucket Quota of 1GB, in a 10 node cluster there would be an aggregate bucket quota of 10GB across the cluster. Adding two nodes to the cluster would extend your aggregate bucket quota to 12GB.
From this description and diagram, you can see that adding new nodes to the cluster expands the overal RAM quota, and the bucket quota, increasing the amount of information that can be kept in RAM.
The Bucket Quota is used by the system to determine when data should be ejected from memory. Bucket Quotas are dynamically configurable within the limit of your Server Quota, and enable you to individually control the caching of information in memory on a per bucket basis. You can therefore configure different buckets to cope with your required caching RAM allocation requirements.
The Server Quota is also dynamically configurable, but care must be taken to ensure that the nodes in your cluster have the available RAM to support your chosen RAM quota configuration.
For more information on changing Couchbase Quotas, see Section 5.9, “Changing Couchbase Quotas”.