[LU-7756] oss_num_threads max value is sometimes too low to feed disk controllers

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.9.0
• Affects Version/s: Lustre 2.5.3
• Labels: patch performance
• Severity: 3
• Assignee: Bob Glossman (Inactive)
• Reporter: Gregoire Pichon

Description

When submitting read IOs from lustre clients to oss which uses 6 OSTs, we see in iostat that the number of IO requests in progress on each lun does not go beyond 86.

The result is that throughput per lun is limited to ~600 MB/s.

To be able to get the most out of the pun we need at least 100 requests at once.

This is due to the max number of oss_num_threads being limited by OSS_NTHRS_MAX (512 / 6 = ~86).

Raising the limit to 1024 by patching the code allowed to get enough I/Os at once on each lun and get up to 900MB/s per OST.