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

On Wed, Jul 15, 2026 at 19:41 +0200, Abd-Alrhman Masalkhi wrote:
> 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...
>

I'm not entirely sure about the media refresh part on modern drives...,
I'll have to look into that first.

>> Regards,
>>
>> Dion
>
> -- 
> Best Regards,
> Abd-Alrhman

-- 
Best Regards,
Abd-Alrhman