Write order fidelity (WOF) is maintained for totally-active
implementations wherein a plurality of access nodes at geographically
separated sites can concurrently read and/or write data in a "totally
active" fashion on a distributed data system. From the hosts' perspective
at diverse geographic locations, a synchronous, cache-coherent view of
data is provided. Data transfer is asynchronous. A time ordered data
image is created and maintained so operations can be restarted after a
partial system failure that causes loss of data not yet asynchronously
transferred across the network, but that has been write-acknowledged to
the originating host. Time ordered asynchronous data transfer is
implemented as a pipeline of changes that reflect contributions from all
nodes. WOF also improves network performance and lowers bandwidth
consumption. Extensions can provide, in a totally-active context,
features such as point-in-time snapshots, time firewalls, on-demand
backend storage allocation, synchronous/asynchronous distribution of
data, and continuous data protection.