Tombstones are records of expired or deleted items that include item keys and metadata.
Couchbase Server and other distributed databases maintain tombstones in order to provide eventual consistency between nodes and between clusters. Tombstones are records of expired or deleted items and they include the key for the item and metadata. Couchbase Server stores the key plus several bytes of metadata per deleted item in two structures per node. With millions of mutations, the space taken up by tombstones can grow quickly. This is especially the case if there are a large number of deletions or expired documents.
The Metadata Purge Interval sets how frequently a node permanently purges metadata on deleted and expired items. The Metadata Purge Interval setting runs as part of auto-compaction. This helps reduce the storage requirement by roughly 3x times lower than before and also frees up space much faster.