Re: Subject: RFC: Read repair for md RAID1 after mirror read failures

Hi Kuai,

On Wed, Jul 15, 2026 at 15:27 +0800, Yu Kuai wrote:
> Hi,
>
> 在 2026/7/15 15:10, G.W. Kant - Hunenet B.V. 写道:
>> On 7/15/26 7:27 AM, Abd-Alrhman Masalkhi wrote:
>>>> In other words, the first successfully recovered read request could
>>>> automatically become a repair opportunity. The repair could even be
>>>> scheduled asynchronously, so the successful read is returned immediately
>>>> while the rewrite is performed in the background. Unlike a periodic
>>>> resync, this repair would be driven by an actual read failure, making it
>>>> targeted rather than rewriting the entire mirror.
>>>>
>>> Yes, md has had this for a long time. Look at fix_read_error() in
>>> raid1.c. It is called from handle_read_error() on any failed read. It
>>> reads from a healthy mirror and rewrites the bad region on the failing
>>> device, giving the drive a chance to rewrite or remap the sector. If the
>>> rewrite fails, it records a bad block. md does this synchronously under
>>> a frozen array, so it is not a missing feature.
>>>
>>> The likely reason you didn't see it is that your array was already
>>> degraded, so there was no healthy in-array copy for fix_read_error() to
>>> recover from. In your case, you were likely able to retrieve the data
>>> due to btrfs level redundancy, and md can't repair across arrays.
>>>
>>>> With today's 18–24 TB HDDs and backup/archive workloads, where data may
>>>> remain unchanged for years, latent media degradation seems increasingly
>>>> relevant. A successful read from the alternate mirror may be one of the
>>>> last opportunities to refresh such a sector before it becomes
>>>> permanently unreadable.
>>>>
>>> And Check/Repair is the right defense for cold archival data on large
>>> drives.
>> Thank you. fix_read_error() was exactly the piece I was looking for. I
>> had missed that md already performs read repair when another healthy
>> mirror is available.
>>
>> My situation was indeed different because the array had already lost one
>> member, so the successful read originated from Btrfs redundancy across
>> another md RAID1 rather than from the surviving md mirror.
>>
>> One question remains, though. check/repair periodically reads all
>> sectors, but as far as I understand it, successfully read sectors are
>> not rewritten. From a media aging perspective this is a different
>> problem than recovering from a read error. Have there ever been
>> discussions about a true media refresh pass that rewrites successfully
>> read sectors to refresh long-lived magnetic recordings?
>
> Perhaps this is what you looking for:
>
> [RFC v2 0/5] md/raid1: introduce a new sync action to repair badblocks - 
> Zheng Qixing 
> <https://lore.kernel.org/all/[email protected]/>
>
> However, AFAIK, Qixing no longer work in this area and there will not be 
> new version.
>

It seems interesting. Do you think it's worth carrying it forward?

>>
>> Best regards,
>> Dion Kant
>
> -- 
> Thanks,
> Kuai

-- 
Best Regards,
Abd-Alrhman