What does --replace actually do?

What does --replace actually do?

[Eyal Lebedinsky]

When I request to replace sda with sdj (RAID6 in my case), does it create sdj from the
other members (NOT using sda) or does it make a copy of (by reading) sda?

I hope it is the former, assuming sda is having issues and the action is precautionary.

I see on stackexchange is mentioned that it actually does the latter:
	http://unix.stackexchange.com/questions/74924/how-to-safely-replace-a-not-yet-failed-disk-in-a-linux-raid5-array

And while I am here, can I replace two members at the same time? This should save time
as it takes about 10 hours to reconstruct a 4TB member.

TIA

-- 
Eyal Lebedinsky (eyal@eyal.emu.id.au)

Re: What does --replace actually do?

[Mikael Abrahamsson]

On Fri, 15 Apr 2016, Eyal Lebedinsky wrote:

> When I request to replace sda with sdj (RAID6 in my case), does it 
> create sdj from the other members (NOT using sda) or does it make a copy 
> of (by reading) sda?

It reads sda and copies it to sdj. When it's done it'll fail sda. If it 
has problems reading from sda, it'll use the rest of the drives to 
recreate/recalculate the block and write that to sdj.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se

Re: What does --replace actually do?

[Robin Hill]

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On Fri Apr 15, 2016 at 09:26:48PM +1000, Eyal Lebedinsky wrote:

> When I request to replace sda with sdj (RAID6 in my case), does it
> create sdj from the other members (NOT using sda) or does it make a
> copy of (by reading) sda?
> 
> I hope it is the former, assuming sda is having issues and the action
> is precautionary.
> 
It's the latter. The data is copied from the existing disk, unless a
read-error occurs, in which case it'll be reconstructed from the other
members. This will be faster and have a far lower impact on performance
than using the other members for everything. If you cannot trust the
disk you're replacing then it should be failed and a normal rebuild
carried out instead.

> And while I am here, can I replace two members at the same time? This
> should save time as it takes about 10 hours to reconstruct a 4TB member.
> 
I've no idea on this one I'm afraid. I can't think of any immediate
reason why this wouldn't be possible, but whether the current
implementation supports it or not is another matter. You could test with
loopback devices.

Cheers,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |

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