[LU-3528] move directory entries cache from llite to MDC

• Type: Technical task
• Resolution: Fixed
• Priority: Major
• Fix Version/s: Lustre 2.6.0, Lustre 2.5.1
• Assignee: Di Wang
• Reporter: Di Wang

Description

This is one of the sub-task for DNE phase 2 striped directory.

Move directory entries cache from llite to MDC, so client side dir stripe can use independent hash function, i.e. it does not need to tight with the backend storage dir-entry hash function. With striped directory, it will be 2-tier hash, LMV calculate hash value according to the name and hash-type in layout, then each MDT will store these entry in disk by its own hash.