Can I replace raid6 disk using dd?

Can I replace raid6 disk using dd?

[Ram Ramesh]

I had a recent disk failure in my 4x2TB raid6 and as a temporary fix I
added a 4tb drive to prevent the drive from being degraded while I
researched for a new drive.

Now that I have purchased a new drive, I am wondering if it is ok to
simply dd the first 2tb of the 4tb drive to replace the drive. I am
asking because my case/motherboard does not have another HD slot to
add the new disk
in parallel with raid6. I was thinking of shutting down the machines,
moving the 4tb memmebr and the new drive to another machine,  making
image copy (only 2tb), and putting the
2tb drive back in the original machine and rebooting. Will this work?

Ramesh

Re: Can I replace raid6 disk using dd?

[Ethan Wilson]

On 18/08/2014 21:26, Ram Ramesh wrote:
> I had a recent disk failure in my 4x2TB raid6 and as a temporary fix I
> added a 4tb drive to prevent the drive from being degraded while I
> researched for a new drive.
>
> Now that I have purchased a new drive, I am wondering if it is ok to
> simply dd the first 2tb of the 4tb drive to replace the drive. I am
> asking because my case/motherboard does not have another HD slot to
> add the new disk
> in parallel with raid6. I was thinking of shutting down the machines,
> moving the 4tb memmebr and the new drive to another machine,  making
> image copy (only 2tb), and putting the
> 2tb drive back in the original machine and rebooting. Will this work?


I think it should work, if metadata is 1.1 or 1.2 . I'm not sure if it 
is 1.0 .
You can also attach it via USB with an adapter, and do a replace (look 
up "want_replacement") then shutdown, swap the drives, and turn on again.

Re: Can I replace raid6 disk using dd?

[Ram Ramesh]

On 08/18/2014 09:30 PM, Ethan Wilson wrote:
> On 18/08/2014 21:26, Ram Ramesh wrote:
>> I had a recent disk failure in my 4x2TB raid6 and as a temporary fix I
>> added a 4tb drive to prevent the drive from being degraded while I
>> researched for a new drive.
>>
>> Now that I have purchased a new drive, I am wondering if it is ok to
>> simply dd the first 2tb of the 4tb drive to replace the drive. I am
>> asking because my case/motherboard does not have another HD slot to
>> add the new disk
>> in parallel with raid6. I was thinking of shutting down the machines,
>> moving the 4tb memmebr and the new drive to another machine, making
>> image copy (only 2tb), and putting the
>> 2tb drive back in the original machine and rebooting. Will this work?
>
>
> I think it should work, if metadata is 1.1 or 1.2 . I'm not sure if it 
> is 1.0 .
> You can also attach it via USB with an adapter, and do a replace (look 
> up "want_replacement") then shutdown, swap the drives, and turn on again.
> -- 
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
I thought about USB. First I do not have one, but that is a small 
problem to overcome. I was thinking more in terms of
performance. USB will slow things down as it is 2.0 (old MB) and dd 
should be real fast (130+ MB, based on the replacement)
But, I agree that mdadm replace is safer. Do I have to remove bitmap for 
replacement? I don't think so, but this is a newer operation and less 
info is available, so making sure.

Ramesh

Re: Can I replace raid6 disk using dd?

[Killian De Volder]

No need to remove the bitmap.
But be very careful that the disk is exactly the same size (or bigger).
I recently had a disk that was slightly smaller then advertised !

But if it's a raid6 why not just jank a disk ?
It's still going to be redundant, but only on 1 disk.

Killian De Volder
Megasoft bvba
killian.de.volder@megasoft.be

On 19-08-14 04:49, Ram Ramesh wrote:
> On 08/18/2014 09:30 PM, Ethan Wilson wrote:
>> On 18/08/2014 21:26, Ram Ramesh wrote:
>>> I had a recent disk failure in my 4x2TB raid6 and as a temporary fix I
>>> added a 4tb drive to prevent the drive from being degraded while I
>>> researched for a new drive.
>>>
>>> Now that I have purchased a new drive, I am wondering if it is ok to
>>> simply dd the first 2tb of the 4tb drive to replace the drive. I am
>>> asking because my case/motherboard does not have another HD slot to
>>> add the new disk
>>> in parallel with raid6. I was thinking of shutting down the machines,
>>> moving the 4tb memmebr and the new drive to another machine, making
>>> image copy (only 2tb), and putting the
>>> 2tb drive back in the original machine and rebooting. Will this work?
>>
>>
>> I think it should work, if metadata is 1.1 or 1.2 . I'm not sure if it is 1.0 .
>> You can also attach it via USB with an adapter, and do a replace (look up "want_replacement") then shutdown, swap the drives, and turn on again.
>> -- 
>> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> I thought about USB. First I do not have one, but that is a small problem to overcome. I was thinking more in terms of
> performance. USB will slow things down as it is 2.0 (old MB) and dd should be real fast (130+ MB, based on the replacement)
> But, I agree that mdadm replace is safer. Do I have to remove bitmap for replacement? I don't think so, but this is a newer operation and less info is available, so making sure.
>
> Ramesh
>
> -- 
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: Can I replace raid6 disk using dd?

[Ethan Wilson]

On 19/08/2014 09:09, Killian De Volder wrote:
> No need to remove the bitmap.
> But be very careful that the disk is exactly the same size (or bigger).
> I recently had a disk that was slightly smaller then advertised !
>
> But if it's a raid6 why not just jank a disk ?
> It's still going to be redundant, but only on 1 disk.
>
> Killian De Volder
> Megasoft bvba
> killian.de.volder@megasoft.be
>

I think it would not be accepted by MD if the size was smaller than 
needed. At least with --replace. With the dd thing I don't know: it 
might trust the metadata and bypass the check.... I'd be careful in that 
case indeed.

Re: Can I replace raid6 disk using dd?

[Ram Ramesh]

On 08/19/2014 04:08 AM, Ethan Wilson wrote:
> On 19/08/2014 09:09, Killian De Volder wrote:
>> No need to remove the bitmap.
>> But be very careful that the disk is exactly the same size (or bigger).
>> I recently had a disk that was slightly smaller then advertised !
>>
>> But if it's a raid6 why not just jank a disk ?
>> It's still going to be redundant, but only on 1 disk.
>>
>> Killian De Volder
>> Megasoft bvba
>> killian.de.volder@megasoft.be
>>
>
> I think it would not be accepted by MD if the size was smaller than 
> needed. At least with --replace. With the dd thing I don't know: it 
> might trust the metadata and bypass the check.... I'd be careful in 
> that case indeed.
> -- 
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
Thank you all. I think many feel dd is risky although doable. I felt the 
same way too. Let me look for alternate/safe ways.

I wanted to experiment because  the real data in the array is small and 
I had good backups. Still, I do not think it is worth risking the chance 
of trashing the array.  I am more worried of the what if scenario, where 
there is corruption, but not detected until my backup is no longer up to 
date - I am not fond of Murphy :-)

Ramesh