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It might be normal and expected, depending how many nodes are in your cluster, how many CPU cores and the amount of RAM you’ve allocated to Couchbase Server node/s. I’d suggest reading our blog post about sizing a cluster. Generally speaking, if you are using 5 buckets, you’ll want to have 8 CPU cores dedicated to Couchbase Server. Each bucket in a Couchbase Server cluster is similar to deploying an entire additional database in a legacy relational database. This CPU usage can be reduced if you were able to reduce the number of buckets that you are using.
I would highly recommend trying out a newer version of Couchbase Server, such as the 7.0 Beta, as we are always improving areas such as resource usage and Couchbase Server 5.1.1 is from Feb 2018 and is actually End Of Life.
Upgrading means you can take advantage of a new feature called Collections in Couchbase Server 7.0. It will allow you to organise and isolate documents in a smaller number of buckets. Collections do not have the resource overhead of individual buckets.
Ian McCloy (Couchbase Principal Product Manager)