Help, my disk array has one dead member

[Bruce Ferrell]

On 03/22/2017 07:17 PM, Xen wrote:
> Bruce Ferrell schreef op 23-03-2017 0:12:
>> On 3/22/17 3:37 PM, Xen wrote:
>>> Bruce Ferrell schreef op 22-03-2017 22:36:
>>>
>>>> Simple raid0 is destroyed with a member drive or partition out so no
>>>> copy is needed/possible.
>>>
>>> He said that one of the drives was damaged, but still readable. The other was fine.
>>>
>> Yes and he also said it's an md raid.  As I said, the devil is in the
>> details;  If it's raid1 (or higher) and one of the elements is
>> damaged, the others cover for that with speed degradation.
>
> He said it was raid0.
>
>> raid0, do NOT use any dd variant on the physical disk.  md has
>> internal data structures on the physical disks and doesn't transplant
>> well to new drives.
>
> He didn't try to do that because there's no point in that if it has a stripe, right.
>
>> You *might* be able to get away with dd on the md device, but that is
>> active and managed by the md subsystem.  I was taught NEVER use dd on
>> active devices... Unless you want trash data.
>
> I don't see a reason against it. The block volume is managed from the outside by the filesystem. The filesystem manages the logical block space. It has no interference with the 
> md subsystem.
>
> In place of the filesystem, you can also use dd on the block device. It makes no difference.
>
> You also can't trash data on any device just by reading, usually.
>
>> Use tar and you may have to be selective about what you copy but then
>> again maybe sector relocation can help you get past the damage for
>> tar.
>
> Maybe, but filesystem recovery is a step after you secure the data, I think.
>
>> Where this get's REALLY dicey; damaged data may have been mirrored
>> back to the good drive from the bad one.  NASTY!
>
> It was a raid0. He said it was a stripe.
>
>> Bottom line, one size never fits all... poke, prod (gently) and use
>> trouble shooting steps to make a determination of what's needed to
>> recover and NEVER blindly follow "just do this..." instructions
>
> It wasn't a blanket just do it instruction. It was geared to his particular use case.
>
> Anyway, sorry for responding again still.
>
the data is NOT secure if you can't restore it ( made a backup to read only media.  why is that bad? ).  it doesn't matter if successfully read every block and can't put it back 
and have it understandable.

I understood it was a stripe.  both raid1 and raid0 are striped. As I said, the devil is in the details.

Which is why I watched the back and forth half the day before responding to any of it.  which is why I gave him steps and theory of operation and pointers to methods so he could be 
enabled to make his own decisions as to next steps.

This stuff is what I DO all day long and have for over 30 years.