When converting a mirror in such a way that adds devices - either to the image count or log - there must be enough devices available to make the change or the command will fail. This is especially pertinent when specifying devices (PVs) to be used for the conversion.
Specified PVs are treated in the following manor when converting:
Up-converting (adding devices to the mirror or its log):
When PVs are specified, they are treated as the only available source for allocation. You must specify enough PVs to satisfy the request.
Down-converting (removing devices from the mirror or its log):
When PVs are specified, they are treated as the preferential devices to be removed. If there are more images to be removed than PVs specified, the remaining images are chosen by LVM.