On Wed, Mar 9, 2022 at 10:48 PM Christoph Hellwig <hch@xxxxxxxxxxxxx> 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
