I think this is the correct answer: The parent directory is stored as a 64-bit object id in znode->zp_parent. This is just an index and is relative to the containing objset. The /index/ remains valid even after the /contents/ of directory ?a? have changed, so directory ?b? does not need updating.
In other words, the objset provides the additional level of indirection required to avoid updating all the dependents such as ?b?. -- This message posted from opensolaris.org
