In vSAN 7.0U1 we introduced something called Enhanced Data Durability for planned maintenances. In vSAN 7.0U2 we expanded the feature to work with unplanned maintenances.
So, what is Enhanced Data Durability? When we put a host in maintenance mode with ensure accessibility, the data components on that host are no longer receiving writes. vSAN creates a durability component, to capture just the new writes. When the host is removed from maintenance mode, the missed writes are synced. This allows for faster resyncs and helps provide additional resiliency in a double fault scenario.
Learn more at http://www.vmware.com/learning
