IBM V7000 Introduction And Implementation Manual page 472

After you create your mirror, you can view the distribution of extents on the assigned mdisks
for each copy or for the entire volume (Figure 10-4). You can also view the mirroring progress
percentage by highlighting the volume and selecting the Volume Synchronization tab in the
Running Tasks radial (Figure 10-5).
Figure 10-4 The distribution of extents for primary and mirror copy of a volume
Figure 10-5 Progress of a mirror copy creation as viewed by using Running Tasks
Extent distribution: Extent distribution for the mirror copy is automatically balanced as
well as possible within the storage pool selected.
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IBM Flex System V7000 Storage Node Introduction and Implementation Guide