For performance, Couchbase Server mainly stores and retrieves information for clients using RAM. At the same time, Couchbase Server will eventually store all data to disk to provide a higher level of reliability. If a node fails and you lose all data in the caching layer, you can still recover items from disk. We call this process of disk storage eventual persistence since the server does not block a client while it writes to disk, rather it writes data to the caching layer and puts the data into a disk write queue to be persisted to disk. Disk persistence enables you to perform backup and restore operations, and enables you to grow your datasets larger than the built-in caching layer. For more information, see Section 1.2.8, “Ejection, Eviction and Working Set Management”.
When the server identifies an item that needs to be loaded from disk because it is not in active memory, the process is handled by a background process that processes the load queue and reads the information back from disk and into memory. The client is made to wait until the data has been loaded back into memory before the information is returned.
Document Deletion from Disk
Couchbase Server will never delete entire items from disk unless a client explicitly deletes the item from the database or the expiration value for the item is reached. The ejection mechanism removes an item from RAM, while keeping a copy of the key and metadata for that document in RAM and also keeping copy of that document on disk. For more information about document expiration and deletion, see Couchbase Developer Guide, About Document Expiration .