On Wed, Mar 9, 2022 at 10:48 PM Christoph Hellwig hch@infradead.org wrote:
On Tue, Mar 08, 2022 at 10:42:09PM -0800, Song Liu wrote:
RAID arrays check/repair operations benefit a lot from merging requests. If we only check the previous entry for merge attempt, many merge will be missed. As a result, significant regression is observed for RAID check and repair.
Fix this by checking more than just the previous entry when plug->multiple_queues == true.
But this also means really significant CPU overhead for all other workloads.
Would the following check help with these workloads?
if (!plug->multiple_queues) break;
This improves the check/repair speed of a 20-HDD raid6 from 19 MB/s to 103 MB/s.
What driver uses multiple queues for HDDs?
Can you explain the workload submitted by a md a bit better? I wonder if we can easily do the right thing straight in the md driver.
It is the md sync_thread doing check and repair. Basically, the md thread reads all the disks and computes parity from data.
Maybe we should add a new flag to struct blk_plug for this special case?
Song