[LU-10278] lfs migrate: make use of direct i/o optional

• Type: Improvement
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.11.0
• Labels: patch
• Assignee: Emoly Liu
• Reporter: Daniel Kobras (Inactive)

Description

In order to avoid cache thrashing, lfs migrate currently uses direct i/o to copy file contents, which may leads to significantly different performance compared to normal (non-O_DIRECT) i/o, cf. for example the discussion at http://lists.lustre.org/pipermail/lustre-discuss-lustre.org/2017-November/015112.html. Internally, we've measured even more extreme differences where O_DIRECT throughput performed at 1/10th the normal rate. Therefore, I'd suggest to leave the choice between normal and direct i/o to the caller of lfs migrate by adding a new option N/-non-direct that suppresses to O_DIRECT flag in the open() call of the source file. Patch to follow.

Issue Links

• is related to: LU-11621 Add copy_file_range() API and use it for lfs migrate and mirror resync
• is related to: LU-4198 Improve IO performance when using DIRECT IO using libaio