A vBucket is defined as the owner of a subset of the key space of a Couchbase cluster. These vBuckets are used to allow information to be distributed effectively across the cluster. The vBucket system is used both for distributing data, and for supporting replicas (copies of bucket data) on more than one node.
vBuckets are not a user-accessible component, but they are a critical component of Couchbase Server and are vital to the availability support and the elastic nature.
Every document ID belongs to a vBucket. A mapping function is used to calculate the vBucket in which a given document belongs. In Couchbase Server, that mapping function is a hashing function that takes a document ID as input and outputs a vBucket identifier. Once the vBucket identifier has been computed, a table is consulted to lookup the server that "hosts" that vBucket. The table contains one row per vBucket, pairing the vBucket to its hosting server. A server appearing in this table can be (and usually is) responsible for multiple vBuckets.
The hashing function used by Couchbase Server to map document IDs to vBuckets is configurable - both the hashing algorithm and the output space (i.e. the total number of vBuckets output by the function). Naturally, if the number of vBuckets in the output space of the hash function is changed, then the table which maps vBuckets to Servers must be resized.