linux-kernel - Re: [PATCH] md/raid1: skip recovery of already synced areas

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Message-ID: <9281d56a-c4ef-9a2b-e06b-12b00e9dccba@huaweicloud.com>
Date: Thu, 18 Sep 2025 16:26:38 +0800
From: Li Nan <linan666@...weicloud.com>
To: John Stoffel <john@...ffel.org>, linan666@...weicloud.com
Cc: song@...nel.org, yukuai3@...wei.com, linux-raid@...r.kernel.org,
 linux-kernel@...r.kernel.org, yangerkun@...wei.com, yi.zhang@...wei.com
Subject: Re: [PATCH] md/raid1: skip recovery of already synced areas



在 2025/9/18 4:05, John Stoffel 写道:
>>>>>> "linan666" == linan666  <linan666@...weicloud.com> writes:
> 
>> From: Li Nan <linan122@...wei.com>
>> When a new disk is added during running recovery, the kernel may
>> restart recovery from the beginning of the device and submit write
>> io to ranges that have already been synchronized.
> 
> Isn't it beter to be safe than sorry?  If the resync fails for some
> reason, how can we be sure the devices really are in sync if you don't
> force the re-write?
> 
>> Reproduce:
>>    mdadm -CR /dev/md0 -l1 -n3 /dev/sda missing missing
>>    mdadm --add /dev/md0 /dev/sdb
>>    sleep 10
>>    cat /proc/mdstat	# partially synchronized
>>    mdadm --add /dev/md0 /dev/sdc
>>    cat /proc/mdstat	# start from 0
>>    iostat 1 sdb sdc	# sdb has io, too
> 
>> If 'rdev->recovery_offset' is ahead of the current recovery sector,
>> read from that device instead of issuing a write. It prevents
>> unnecessary writes while still preserving the chance to back up data
>> if it is the last copy.
> 
> 
> Are you saying that sdb here can continute writing from block N, but
> that your change will only force sdc to start writing from block 0?
> Your description of the problem isn't really clear.
> 
Thanks for your review. You're right, I will describe it more accurately
in the next patch.

> I think it's because you're using the word device for both the MD
> device itself, as well as the underlying device(s) or component(s) of
> the MD device.
> 
> 
> 
>> Signed-off-by: Li Nan <linan122@...wei.com>
>> ---
>>   drivers/md/raid1.c | 6 +++++-
>>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
>> diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
>> index 3e422854cafb..ac5a9b73157a 100644
>> --- a/drivers/md/raid1.c
>> +++ b/drivers/md/raid1.c
>> @@ -2894,7 +2894,8 @@ static sector_t raid1_sync_request(struct mddev *mddev, sector_t sector_nr,
>>   		    test_bit(Faulty, &rdev->flags)) {
>>   			if (i < conf->raid_disks)
>>   				still_degraded = true;
>> -		} else if (!test_bit(In_sync, &rdev->flags)) {
>> +		} else if (!test_bit(In_sync, &rdev->flags) &&
>> +			   rdev->recovery_offset <= sector_nr) {
> bio-> bi_opf = REQ_OP_WRITE;
> bio-> bi_end_io = end_sync_write;

As Yu Kuai mentioned, I will refactor this part of the code later.
This patch will be dropped.

>>   			write_targets ++;
>> @@ -2903,6 +2904,9 @@ static sector_t raid1_sync_request(struct mddev *mddev, sector_t sector_nr,
>>   			sector_t first_bad = MaxSector;
>>   			sector_t bad_sectors;
>   
>> +			if (!test_bit(In_sync, &rdev->flags))
>> +				good_sectors = min(rdev->recovery_offset - sector_nr,
>> +						   (u64)good_sectors);
>>   			if (is_badblock(rdev, sector_nr, good_sectors,
>>   					&first_bad, &bad_sectors)) {
>>   				if (first_bad > sector_nr)
>> -- 
>> 2.39.2
> 
> 
> 
> .

-- 
Thanks,
Nan