IBM System storage DS6000 Series Redbook page 287

If this persists for a significant period of time, then eventually Global Mirror will force the
formation of a new Consistency Group. In this way Global Mirror ensures that during
periods when the bandwidth is insufficient, production performance is protected and data
is transmitted to the secondary site in the most efficient manner possible. When the peak
activity has passed, consistency group formation will resume in a timely fashion.
Consistency Group interval time: once a Consistency Group has been created, the CG
interval time determines how long to wait before starting with the formation of the next
Consistency Group. This is specified in seconds, and the default is zero (0) seconds. Zero
seconds means that Consistency Group formation happens constantly. As soon as a
Consistency Group is successfully created, the process to create a new Consistency
Group starts again immediately.
There is no external parameter to limit the time for the FlashCopy operation. This is due to its
very fast bitmap manipulation process.
Global Mirror utilizes a distributed approach as well as a two-phase commit technique for
activities between the Master and its Subordinate LSSs. The communication between the
local and the remote site is organized through the Subordinate LSS. The Subordinates
function here partly as a transient component for the Global Mirror activities, which are all
triggered and coordinated by the Master. This distributed concept allows always to provide a
set of data consistent volumes at the remote site independent of the number of involved
storage disk subsystems at the primary or at the secondary site.
Chapter 22. Global Mirror overview
263