During an extended distance operation, the ESS captures information about updates to the primary and periodically sends those updates to the secondary. After the initial copy of tracks, the ESS periodically starts a synchronization cycle where all updated tracks, in ascending order from the lowest numbered track, is copied from the primary volume to the secondary volume.
The ESS updates the secondary tracks with the current information for that track, regardless of the number of updates between the time it was last copied, the current time, and the order in which the updates occurred. When this process completes, the cycle is repeated.
There is little response time degradation on application I/O while operating in extended distance mode. Write updates to the primary volume receive an immediate completion because the synchronization cycle is independent of the primary write updates.