Couchbase Server mainly stores and retrieves information for clients using RAM. At the same time, Couchbase Server eventually stores all data to disk to provide a higher level of reliability.
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 to grow your datasets larger than the built-in caching layer. This disk storage process is called eventual persistence since the server does not block a client while it writes to disk.
If a node fails and all data in the caching layer is lost, the items are recovered from disk. 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 waits until the data has been loaded back into memory before the information is returned.