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

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Message-ID: <0106019b9349bf39-5460c782-3d63-43cb-a56e-e34760ce51cd-000000@ap-northeast-1.amazonses.com>
Date: Tue, 6 Jan 2026 12:30:42 +0000
From: Kenta Akagi <k@...l.me>
To: linan666@...weicloud.com, xni@...hat.com
Cc: linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org, 
	song@...nel.org, yukuai@...as.com, shli@...com, mtkaczyk@...nel.org
Subject: Re: [PATCH v6 1/2] md: Don't set MD_BROKEN for RAID1 and RAID10
 when using FailFast

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

> succeed".
> 
> -- 
> Thanks,
> Nan
> 
>