Re: [PATCH md-6.9 v4 00/11] md/raid1: refactor read_balance() and some minor fix

[Song Liu]

On Thu, Feb 29, 2024 at 2:03 AM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> Changes in v4:
>  - fix a problem in v2, that replacement rdev->raid_disk is set to
>  raid_disk + conf->mirros, which will cause test 01replace to run
>  forever, and mdadm tests looks good now(no new regression);
> Changes in v3:
>  - add patch 2, and fix that setup_conf() is missing in patch3;
>  - add some review tag from Xiao Ni(other than patch 2,3);
> Changes in v2:
>  - add new conter in conf for patch 2;
>  - fix the case choose next idle while there is no other idle disk in
>  patch 3;
>  - add some review tag from Xiao Ni for patch 1, 4-8
>
> The original idea is that Paul want to optimize raid1 read
> performance([1]), however, we think that the original code for
> read_balance() is quite complex, and we don't want to add more
> complexity. Hence we decide to refactor read_balance() first, to make
> code cleaner and easier for follow up.
>
> Before this patchset, read_balance() has many local variables and many
> branches, it want to consider all the scenarios in one iteration. The
> idea of this patch is to divide them into 4 different steps:
>
> 1) If resync is in progress, find the first usable disk, patch 5;
> Otherwise:
> 2) Loop through all disks and skipping slow disks and disks with bad
> blocks, choose the best disk, patch 10. If no disk is found:
> 3) Look for disks with bad blocks and choose the one with most number of
> sectors, patch 8. If no disk is found:
> 4) Choose first found slow disk with no bad blocks, or slow disk with
> most number of sectors, patch 7.
>
> Note that step 3) and step 4) are super code path, and performance
> should not be considered.
>
> And after this patchset, we'll continue to optimize read_balance for
> step 2), specifically how to choose the best rdev to read.
>
> [1] https://lore.kernel.org/all/20240102125115.129261-1-paul.e.luse@xxxxxxxxxxxxxx/
>
> Yu Kuai (11):
>   md: add a new helper rdev_has_badblock()
>   md/raid1: factor out helpers to add rdev to conf
>   md/raid1: record nonrot rdevs while adding/removing rdevs to conf
>   md/raid1: fix choose next idle in read_balance()
>   md/raid1-10: add a helper raid1_check_read_range()
>   md/raid1-10: factor out a new helper raid1_should_read_first()
>   md/raid1: factor out read_first_rdev() from read_balance()
>   md/raid1: factor out choose_slow_rdev() from read_balance()
>   md/raid1: factor out choose_bb_rdev() from read_balance()
>   md/raid1: factor out the code to manage sequential IO
>   md/raid1: factor out helpers to choose the best rdev from
>     read_balance()

Applied v4 of the set to md-6.9 branch.

Thanks,
Song