[LU-15971] Implicit default LMV inherit

• Type: New Feature
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.16.0
• Assignee: Lai Siyao
• Reporter: Lai Siyao

Description

With implicit default LMV inherit, the inherited default LMV is maintained on client side.

Benefits:

• change of directory default LMV will be populated to all sub levels at runtime automatically.
• default LMV will be packed into mkdir request, therefore MDT doesn't need read it from disk, as will improve mkdir performance.

Caveats:

• old and new clients can't coexist, because after new client mkdir,
  the inherited default LMV is not set on MDT inode, then the old
  client won't know its existence.

Changes on client side:

• update default LMV after lookup/open/revalidate.
• pack default LMV in mkdir request.

Changes on MDT side:

• use the default LMV from client in lod_ah_init().
• don't save inherited default LMV in mkdir.