question about how to incrementally rebuild an image out of cluster

["=?gb18030?b?0KG93A==?=" <jaychj@xxxxxxxxxx>]

The rbd diff-related commands compare points in time of a single
image. Since children are identical to their parent when they're cloned,
if I created a snapshot right after it was cloned, I could export
the diff between the used child and the parent. Something like:

rbd clone child parent@snap
rbd snap create child@base
<use child image>
rbd snap create child@changed
rbd export-diff child@changed --from-snap base child_changes.diff

the questions are:
1. Does the child@base really necessary? It seems point the same obj with parent@snap, why should'n I use the parent@snap instead?
2. If I create child@base, comparing with use child image directly, are there some performance redundant or storage increment when writing the child image later?ÿôèº{.nÇ+‰·Ÿ®‰­†+%ŠËÿ±éݶ¥Šwÿº{.nÇ+‰·œz˜ÿuëÞ—ø§¶›¡Ü¨}©ž²Æ zÚ&j:+v‰¨þø¯ù®w¥þŠà2ŠÞ™¨è­Ú&¢)ß¡«a¶Úÿÿûàz¿äz¹Þ—ú+ƒùšŽŠÝ¢jÿŠwèþf