[LU-16611] default read_cache_enable=0 not always the best choice when rotational=0

• Type: Improvement
• Resolution: Unresolved
• Priority: Minor
• Affects Version/s: Lustre 2.12.9, Lustre 2.15.2
• Environment: osd-ldiskfs
• Assignee: WC Triage
• Reporter: Stephane Thiell

Description

read_cache_enable is disabled by default if queue/rotational=0

I wanted to raise awareness that this might not be the best default value. I recently noticed that, at least on recent hardware (eg. AMD EPYC Milan) with SAS-based SSDs (not NVMe), it is more efficient to keep read_cache_enable enabled to improve the read performance with many fio benchmarks I ran (sequential I/Os and random I/Os), at least the ones that could benefit from OSS caching (I was careful to not benchmark the client cache...).

This is at least my experience with SAS SSDs, but I understand it might be better to keep the OSS read cache disabled with NVMe drives when accessed directly.