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

Hi Dion,

On Wed, Jul 15, 2026 at 14:13 +0000, G. W. Kant wrote:
> Hi Abd,
>
> On 7/15/26 7:27 AM, Abd-Alrhman Masalkhi wrote:
>>> 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.
>
>
> I have been looking into your suggestion that periodic check/repair is 
> the appropriate defense for cold archival data on large drives.
>
> Before I start reading the resync code, I would like to make sure I 
> understand what repair actually does.
>
> Suppose all mirrors read successfully and contain identical data. In 
> other words, there are no read errors and no mismatches.
>
> Does repair simply verify the data and move on, or does it also rewrite 
> each successfully read block to the corresponding LBA on every mirror?
>
> The distinction is important for what I have in mind.
>
> My concern is not recovery from an existing read error—that is already 
> handled by fix_read_error()—but rather the long-term aging of cold 
> magnetic recordings.
>
> If a sector is still readable today but is approaching the point where 
> the drive's ECC will eventually no longer be able to reconstruct it, 
> then a successful rewrite would effectively establish a new "write time" 
> for that sector. If the drive can no longer reliably store the data at 
> that location, the rewrite would also give the firmware an opportunity 
> to remap the sector.
>
> So my question is essentially:
>
> Does repair perform such a rewrite of successfully read blocks, or does 
> it only rewrite blocks when a read error or mismatch has already been 
> detected?
>
Okay, I see what you're getting at now. No, check/repair won't rewrite
sectors that are read successfully and match across all mirrors. It only
rewrites data when it encounters a mismatch or needs to recover from a
read error.

I think a full-array media refresh is a reasonable idea. but to my
knowlage, modern drives already perform background media scans that
refresh or remap marginal sectors, which covers part of this...

> Regards,
>
> Dion

-- 
Best Regards,
Abd-Alrhman