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Subject: Re: [PATCH] md/raid10: prioritize adding disk to 'removed' mirror
To:     linan666@huaweicloud.com, song@kernel.org
Cc:     linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org,
        linan122@huawei.com, yi.zhang@huawei.com, houtao1@huawei.com,
        yangerkun@huawei.com, "yukuai (C)" <yukuai3@huawei.com>
References: <20230527092007.3008856-1-linan666@huaweicloud.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <45ee7cb9-6651-46a5-87a2-dd66532084d7@huaweicloud.com>
Date:   Mon, 29 May 2023 21:00:49 +0800
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Hi,

?? 2023/05/27 17:20, linan666@huaweicloud.com д??:
> From: Li Nan <linan122@huawei.com>
> 
> When add a new disk to raid10, it will traverse conf->mirror from start
> and find one of the following mirror to add:
>    1. mirror->rdev is set to WantReplacement and it have no replacement,
>       set new disk to mirror->replacement.
>    2. no mirror->rdev, set new disk to mirror->rdev.
> 
> There is a array as below (sda is set to WantReplacement):
> 
>      Number   Major   Minor   RaidDevice State
>         0       8        0        0      active sync set-A   /dev/sda
>         -       0        0        1      removed
>         2       8       32        2      active sync set-A   /dev/sdc
>         3       8       48        3      active sync set-B   /dev/sdd
> 
> Use 'mdadm --add' to add a new disk to this array, the new disk will
> become sda's replacement instead of add to removed position, which is
> confusing for users. Meanwhile, after new disk recovery success, sda
> will be set to Faulty.
> 
> Prioritize adding disk to 'removed' mirror is a better choice. In the
> above scenario, the behavior is the same as before, except sda will not
> be deleted. Before other disks are added, continued use sda is more
> reliable.
> 

I think this change make sense, however, it's better to do this for all
personality instead of just for raid10.

Thanks,
Kuai
> Signed-off-by: Li Nan <linan122@huawei.com>
> ---
>   drivers/md/raid10.c | 35 ++++++++++++++++++++---------------
>   1 file changed, 20 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
> index 4fcfcb350d2b..d90eb830ca1a 100644
> --- a/drivers/md/raid10.c
> +++ b/drivers/md/raid10.c
> @@ -2148,9 +2148,10 @@ static int raid10_add_disk(struct mddev *mddev, struct md_rdev *rdev)
>   {
>   	struct r10conf *conf = mddev->private;
>   	int err = -EEXIST;
> -	int mirror;
> +	int mirror, repl_slot = -1;
>   	int first = 0;
>   	int last = conf->geo.raid_disks - 1;
> +	struct raid10_info *p;
>   
>   	if (mddev->recovery_cp < MaxSector)
>   		/* only hot-add to in-sync arrays, as recovery is
> @@ -2173,23 +2174,14 @@ static int raid10_add_disk(struct mddev *mddev, struct md_rdev *rdev)
>   	else
>   		mirror = first;
>   	for ( ; mirror <= last ; mirror++) {
> -		struct raid10_info *p = &conf->mirrors[mirror];
> +		p = &conf->mirrors[mirror];
>   		if (p->recovery_disabled == mddev->recovery_disabled)
>   			continue;
>   		if (p->rdev) {
> -			if (!test_bit(WantReplacement, &p->rdev->flags) ||
> -			    p->replacement != NULL)
> -				continue;
> -			clear_bit(In_sync, &rdev->flags);
> -			set_bit(Replacement, &rdev->flags);
> -			rdev->raid_disk = mirror;
> -			err = 0;
> -			if (mddev->gendisk)
> -				disk_stack_limits(mddev->gendisk, rdev->bdev,
> -						  rdev->data_offset << 9);
> -			conf->fullsync = 1;
> -			rcu_assign_pointer(p->replacement, rdev);
> -			break;
> +			if (test_bit(WantReplacement, &p->rdev->flags) &&
> +			    p->replacement == NULL && repl_slot < 0)
> +				repl_slot = mirror;
> +			continue;
>   		}
>   
>   		if (mddev->gendisk)
> @@ -2206,6 +2198,19 @@ static int raid10_add_disk(struct mddev *mddev, struct md_rdev *rdev)
>   		break;
>   	}
>   
> +	if (err && repl_slot >= 0) {
> +		p = &conf->mirrors[repl_slot];
> +		clear_bit(In_sync, &rdev->flags);
> +		set_bit(Replacement, &rdev->flags);
> +		rdev->raid_disk = repl_slot;
> +		err = 0;
> +		if (mddev->gendisk)
> +			disk_stack_limits(mddev->gendisk, rdev->bdev,
> +					  rdev->data_offset << 9);
> +		conf->fullsync = 1;
> +		rcu_assign_pointer(p->replacement, rdev);
> +	}
> +
>   	print_conf(conf);
>   	return err;
>   }
>