Re: Why not just return an error?

[Wols Lists <antlists@youngman.org.uk>]

On 11/10/16 05:00, Brad Campbell wrote:
> On 11/10/16 05:55, Wols Lists wrote:
>> On 10/10/16 22:37, Andreas Klauer wrote:
>>> On Mon, Oct 10, 2016 at 09:47:04PM +0100, Anthony Youngman wrote:
>>>> with a list of all blocks that failed to copy. Then we need to patch
>>>> the
>>>> low-level disk access code so that it reads this list of "bad blocks"
>>>> and returns a read error if any attempt is made to read one. If a block
>>>
>>> hdparm has that feature to mark sectors as bad (--make-bad-sector).
>>> not sure how that behaves on a re-write by md. I never tried it myself.
>>
>> I'm guessing it's useless ...
> 
> Not useless at all.

Ahh...
> 
>> The point is that the disk sector is not bad. So you don't want to mark
>> it as bad on the disk. But you know that the *data* in that block is
>> bad, so you want the disk access layer to fake a read error when you try
>> to read it. The intent is to deliberately trigger a rewrite by md.
> 
> I suggested this a while ago. Take the badblocks log, use hdparm to mark
> each bad sector as bad and put the drive back in the array. I even
> suggested potentially adding a feature to ddrescue to auto-mark the
> blocks as bad on the target drive.

But does that mean that the drive thinks those sectors are bad, and that
they're then lost permanently at the hardware level? That's what I
thought the badblocks list did with hdparm, and that's what I was trying
to avoid.
> 
> When md reads from that bad sector it will get an immediate error from
> the drive, reconstruct the data and rewrite it, clearing the bad sector.
> 
> This absolutely prevents a rescued disk from returning zeros rather than
> bad data, and allows the good parts of the disk to participate in the
> array redundancy while stuff gets rectified.
> 
> This is only useful where you have several dud disks in an array and the
> bad sectors are not in the same stripes, but for that pathological case
> it would allow ddrescuing onto new drives and reconstructing the array
> without data loss. Whereas simply using ddrescued disks will happily
> return zeros where the holes are.
> 
My thoughts exactly :-) Indeed, it's probably you I got the idea from :-)

Cheers,
Wol