Why do I need reserved capacity for each member volume?
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Each member volume in a snapshot consistency group must have its own reserved capacity to save any modifications made by the host application to the base volume without affecting the referenced consistency group snapshot image. Reserved capacity provides the host application with write access to a copy of the data contained in the member volume that is designated as read-write.
A consistency group snapshot image is not directly read or write accessible to hosts. Rather, the snapshot image is used to save only the data captured from the base volume.
During the creation of a consistency group snapshot volume that is designated as read-write, System Manager creates a reserved capacity for each member volume in the consistency group. This reserved capacity provides the host application with write access to a copy of the data contained in the consistency group snapshot image.