[LU-19015] Possible records skipping during changelog processing when an ENOSPC occurred while writing a record

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.17.0
• Severity: 3
• Assignee: Alexander Boyko
• Reporter: Alexander Boyko

Description

Multiple threads get into mdd_changelog_write_rec(), they all get their own offsets and now start dt_record_write() in parallel.
Due to scheduling one thread with an offset corresponding to block X+1 starts first and allocates block X+1, few more threads now can write to X+1.
Another thread with block X can't allocate it due to ENOSPC.

This situation leads to a sparse file for the changelog. When processing, dt_read() will get a zeroed block for an offset that has not been written. And llog_process_thread() will skip an entire chunk (2 blocks), losing changelog records from the second block. I understand that there is a small chance of this happening, but it is still possible that regression could occur after LU-18218.

Issue Links

• is related to: LU-18218 Performance impact on MDT performance with changelogs enabled