[LU-15481] speedup "lfs changelog" llog processing with a startrec

• Type: Improvement
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.16.0, Lustre 2.15.4
• Labels: changelog llog
• Assignee: Etienne Aujames
• Reporter: Etienne Aujames

Description

Accessing a changelog record at the end of a catalog could be really slow if there is a lot of "idle records" or if difference of indexes between changelog users is important.
In all cases, changelog llog callback (chlg_read_cat_process_cb) is called for each changelog records.

We could easily determine the number of record to skip based on internal changelog indexes. This enable to conditionally skip an entire llog plain file (64k records) (see LU-14688).