On 11/10/16 05:00, Brad Campbell wrote:

On 11/10/16 05:55, Wols Lists wrote:

quoted

On 10/10/16 22:37, Andreas Klauer wrote:

quoted

On Mon, Oct 10, 2016 at 09:47:04PM +0100, Anthony Youngman wrote:

quoted

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...

quoted

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