[LU-9618] Connect readahead to prep_partial_page to improve small (< 1 page) write performance

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.11.0
• Labels: patch
• Severity: 3
• Assignee: Patrick Farrell
• Reporter: Patrick Farrell

Description

When writing to a region of a file less than file size (either an already existing file or a shared file with multiple writers), writes of less than one page in size must first read in that page.

This results in extremely poor performance. For the random I/O case, there's no easy improvements available, but the sequential case can benefit enormously by using readahead to bring in those pages.

This can be done by using the normal read code w/readahead from the ll_prepare_partial_page function. This works well in the sequential case (3-10x performance improvement) and has no effect on the random case.

Patch & benchmark #s forthcoming shortly.

Issue Links

• is related to: LU-12718 Readahead can trigger an assertion failire in cl_io_read_ahead
• is related to: LU-11140 unassume cl page upon write error
• is related to: LU-8964 use parallel I/O to improve performance on machines with slow single thread performance