[LU-12988] improve mount time on huge ldiskfs filesystem

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Fixed
• Fix Version/s: Lustre 2.14.0
• Labels: LTS12
• Assignee: Alex Zhuravlev
• Reporter: Alex Zhuravlev

Description

during Lustre server startup few small files need to be updated (e.g. config backup).
at this point buddy/bitmap cache is empty but mballoc wants to find a big chunk of free space for group preallocation and reads bitmaps one by one.
sometimes this can take a very long time.
one possisble workaround is to disable preallocation during mount.
a long term plan is to limit scanning and prefetch bitmaps, but this needs more efforts and will be tracked separately (LU-12970)

Issue Links

• is related to: LU-13290 Write performance regression in ldiskfs patches
• is related to: LU-12970 improve mballoc for huge filesystems
• is related to: LU-12103 Improve block allocation for large partitions