On 06/03/2010 02:19 AM, Neil Brown wrote:

On Wed, 02 Jun 2010 18:25:58 +0200
Nicolas Jungers[off-list ref]  wrote:

quoted

I've a 4 HD raid10 with to failed drive.  Any attempt I made to add 2
replacement disks fail consistently.

[snip]

If one of these is actually usable and just had a transient failure then you
could try re-creating the array with the drives, or 'missing' in the right
order and with the write layout/chunksize set.
You would need to be user the 'Data Offset' was the same, which unfortunately
can require using exactly the same version of mdadm as created the array in
the first place.

I managed to copy the two failed disk on a new one (same brand/model) 
with (gnu) ddrescue for a grand total of 512 B lost.  With that copy and 
a copy of one of the non failed disk I recreated (mdadm -C) the array 
over the disks with the same creation parameters and two missing drives. 
  I'm not sure that the procedure was quicker than pulling the data back 
from the backup, but nevertheless, the exercise was interesting.

When thinking about it, could it not be automated/detected in some way 
by mdadm or a related utility?  Or documented in a FAQ?  I had the 
feeling that the close to easy recovery state could be eased by mdadm 
itself, or am I dreaming?

N.

NeilBrown

quoted

mdadm --examine /dev/sdm2
/dev/sdm2:
            Magic : a92b4efc
          Version : 1.2
      Feature Map : 0x0
       Array UUID : d90ad6fe:1355134f:f83ffadc:a4fe7859
             Name : m1:1
    Creation Time : Thu Apr  1 21:28:58 2010
       Raid Level : raid10
     Raid Devices : 4

   Avail Dev Size : 3907026909 (1863.02 GiB 2000.40 GB)
       Array Size : 7814049792 (3726.03 GiB 4000.79 GB)
    Used Dev Size : 3907024896 (1863.01 GiB 2000.40 GB)
      Data Offset : 272 sectors
     Super Offset : 8 sectors
            State : clean
      Device UUID : e217355e:632ac2f0:8120e55e:3878bd88

      Update Time : Wed Jun  2 12:31:39 2010
         Checksum : feef2809 - correct
           Events : 1377156

           Layout : near=2, far=1
       Chunk Size : 1024K

      Array Slot : 3 (failed, failed, 2, 3)
     Array State : __uU 2 failed
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