Re: [PATCH v6 1/2] md: Don't set MD_BROKEN for RAID1 and RAID10 when using FailFast

[Li Nan <linan666@huaweicloud.com>]

在 2026/1/6 20:30, Kenta Akagi 写道:
> Hi,
> Thank you for reviewing.
> 
> On 2026/01/06 11:57, Li Nan wrote:
>>
>>
>> 在 2026/1/5 22:40, Kenta Akagi 写道:
>>> After commit 9631abdbf406 ("md: Set MD_BROKEN for RAID1 and RAID10"),
>>> if the error handler is called on the last rdev in RAID1 or RAID10,
>>> the MD_BROKEN flag will be set on that mddev.
>>> When MD_BROKEN is set, write bios to the md will result in an I/O error.
>>>
>>> This causes a problem when using FailFast.
>>> The current implementation of FailFast expects the array to continue
>>> functioning without issues even after calling md_error for the last
>>> rdev.  Furthermore, due to the nature of its functionality, FailFast may
>>> call md_error on all rdevs of the md. Even if retrying I/O on an rdev
>>> would succeed, it first calls md_error before retrying.
>>>
>>> To fix this issue, this commit ensures that for RAID1 and RAID10, if the
>>> last In_sync rdev has the FailFast flag set and the mddev's fail_last_dev
>>> is off, the MD_BROKEN flag will not be set on that mddev.
>>>
>>> This change impacts userspace. After this commit, If the rdev has the
>>> FailFast flag, the mddev never broken even if the failing bio is not
>>> FailFast. However, it's unlikely that any setup using FailFast expects
>>> the array to halt when md_error is called on the last rdev.
>>>
>>
>> In the current RAID design, when an IO error occurs, RAID ensures faulty
>> data is not read via the following actions:
>> 1. Mark the badblocks (no FailFast flag); if this fails,
>> 2. Mark the disk as Faulty.
>>
>> If neither action is taken, and BROKEN is not set to prevent continued RAID
>> use, errors on the last remaining disk will be ignored. Subsequent reads
>> may return incorrect data. This seems like a more serious issue in my opinion.
> 
> I agree that data inconsistency can certainly occur in this scenario.
> 
> However, a RAID1 with only one remaining rdev can considered the same as a plain
> disk. From that perspective, I do not believe it is the mandatory responsibility
> of md raid to block subsequent writes nor prevent data inconsistency in this situation.
> 
> The commit 9631abdbf406 ("md: Set MD_BROKEN for RAID1 and RAID10") that introduced
> BROKEN for RAID1/10 also does not seem to have done so for that responsibility.
> 
>>
>> In scenarios with a large number of transient IO errors, is FailFast not a
>> suitable configuration? As you mentioned: "retrying I/O on an rdev would
> 
> It seems be right about that. Using FailFast with unstable underlayer is not good.
> However, as md raid, which is issuer of FailFast bios,
> I believe it is incorrect to shutdown the array due to the failure of a FailFast bio.
> 
> Thanks,
> Akagi
> 

I get your point, Kuai, what's your take on this?

-- 
Thanks,
Nan