Neil Brown wrote:

On Wednesday June 25, maan@systemlinux.org wrote:
  

quoted

On 15:37, Dave Moon wrote:

    

quoted

1. If mdadm encounters a bit error during a RAID 6 rebuild, will it  
just give up on that particular file and move on to recover other data  
on the array? Or will it trash the entire array?
      

The kernel will stop the array and give up.
    

Not quite.  It will stop the recovery.  It won't stop the whole array
though (I think...).

  

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2. Is it possible to cheat mdadm by somehow replacing the new "raid  
metadata" on the 6 drives with the old data on the 2 drives? Will it  
make mdadm think the array is clean, consistent and nothing ever  
happened?
      
Please do note that I did not write ANY new data onto the RAID 6 array  
from the time it was degraded until the time I brought it down with (-- 
stop).
      

Use --force, Luke. Man mdadm(8):

	-f, --force Assemble the array even if some superblocks
	appear out-of-date
    

--force only updates enough superblocks to assemble a working array.
For raid6, that mean n-2 drives.  As there are n-2 drive, it won't try
any harder.

You best bet is to recreate the array with --assume-clean.
Providing you have the chunksize, order of devices, etc the same, you
should get your array back.
  

Then what? What's going to happen when he does a check?

Of course build an array out of drives so unstable that you can't safelt 
*run* a check is another topic.

-- 
Bill Davidsen [off-list ref]
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark