For the directory tree based filesystem namespace, the parent directory references the child directory via some unique name entry, and the child directory back references its parent via its unique ".." name entry. Such pairs should match each other.

Usually, for local filesystem or Lustre backend system, such as ldiskfs, the local filesystem consistency verification tools, such as e2fsck can guarantee such consistency. But for distributed system, for example, the Lustre with multiple MDTs under DNE mode, the parent directory and the child directory can reside on different MDTs. The local filesystem consistency verification tools cannot guarantee such consistency as does for local cases.

The LFSCK 3 should check and repair the inconsistency between the cross-MDTs parent directory and child directory pairs.