The difference is both Cassandra and Mongo guarantee that the data on disk is never behind by more than the sync interval (i.e. if the disk gets behind, writes

The difference is both Cassandra and Mongo guarantee that the data on disk is never behind by more than the sync interval (i.e. if the disk gets behind, writes block). Couchbase does not do this; it syncs periodically, but if the disk gets behind writes continue at the same rate, so the only limit to how much data can be lost is the size of your RAM.