Your change rate needs to include only the condition of the data at the point-in-time that the  image was taken, rather than all the updates during the period. The use of this function can  provide significant reductions in replication volume.  Global Mirror with Change Volumes has the following characteristics: Figure 11-92 shows a simple Global Mirror relationship without Change Volumes. With Change Volumes, this environment looks as it is shown in Figure 11-93.  With Change Volumes, a FlashCopy mapping exists between the primary volume and the  primary change volume. The mapping is updated in the cycling period (60 seconds - 1 day).  The primary change volume is then replicated to the secondary Global Mirror volume at the  target site, which is then captured in another change volume on the target site. This approach  provides an always consistent image at the target site and protects your data from being  inconsistent during resynchronization.  For more information about IBM FlashCopy, see 11.1, “IBM FlashCopy” on page 460. 556  Implementing the IBM System Storage SAN Volume Controller with IBM Spectrum Virtualize V8.2.1 Figure 11-93 Global Mirror with Change Volumes Figure 11-92 Global Mirror without Change Volumes GLYPH<SM590000> Larger RPO GLYPH<SM590000> Point-in-time copies GLYPH<SM590000> Asynchronous GLYPH<SM590000> Possible system performance resource requirements because point-in-time copies are  created locally