Re: md raid recovery - perplexed

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On Fri, 2012-04-20 at 18:48 -0600, Ken Gunderson wrote:
> Hello List:
> 
> I've created some arrays.  For example, md2 is RAID1 created with gpt
> based partitions  /dev/sd[ab]1
> 
> # mdadm --misc --detail /dev/md2
> 
> /dev/md2:
>         Version : 1.0
>   Creation Time : Thu Apr 19 15:56:18 2012
>      Raid Level : raid1
>      Array Size : 262132 (256.03 MiB 268.42 MB)
>   Used Dev Size : 262132 (256.03 MiB 268.42 MB)
>    Raid Devices : 2
>   Total Devices : 2
>     Persistence : Superblock is persistent
> 
>     Update Time : Fri Apr 20 09:08:11 2012
>           State : clean 
>  Active Devices : 2
> Working Devices : 2
>  Failed Devices : 0
>   Spare Devices : 0
> 
>            Name : archiso:2
>            UUID : e3a5c30e:3fb61039:397992ff:6cc70600
>          Events : 17
> 
>     Number   Major   Minor   RaidDevice State
>        0       8        1        0      active sync   /dev/sda1
>        1       8       17        1      active sync   /dev/sdb1
> 
> 
> Okay, great, that works.  However, I am not able to recover from
> simulated failure.
> 
> # mdadm /dev/md2 --fail /dev/sdb1
> 
> # mdadm /dev/md2 --misc --detail
> 
> /dev/md2:
>         Version : 1.0
>   Creation Time : Thu Apr 19 15:56:18 2012
>      Raid Level : raid1
>      Array Size : 262132 (256.03 MiB 268.42 MB)
>   Used Dev Size : 262132 (256.03 MiB 268.42 MB)
>    Raid Devices : 2
>   Total Devices : 2
>     Persistence : Superblock is persistent
> 
>     Update Time : Fri Apr 20 15:40:10 2012
>           State : clean, degraded 
>  Active Devices : 1
> Working Devices : 1
>  Failed Devices : 1
>   Spare Devices : 0
> 
>            Name : archiso:2
>            UUID : e3a5c30e:3fb61039:397992ff:6cc70600
>          Events : 20
> 
>     Number   Major   Minor   RaidDevice State
>        0       8        1        0      active sync   /dev/sda1
>        1       0        0        1      removed
> 
>        1       8       17        -      faulty spare   /dev/sdb1
> 
> 
> 
> Followed by
> 
> # mdadm /dev/md2 --remove /dev/sdb1
> 
> /dev/md2:
>         Version : 1.0
>   Creation Time : Thu Apr 19 15:56:18 2012
>      Raid Level : raid1
>      Array Size : 262132 (256.03 MiB 268.42 MB)
>   Used Dev Size : 262132 (256.03 MiB 268.42 MB)
>    Raid Devices : 2
>   Total Devices : 1
>     Persistence : Superblock is persistent
> 
>     Update Time : Fri Apr 20 15:59:52 2012
>           State : clean, degraded 
>  Active Devices : 1
> Working Devices : 1
>  Failed Devices : 0
>   Spare Devices : 0
> 
>            Name : archiso:2
>            UUID : e3a5c30e:3fb61039:397992ff:6cc70600
>          Events : 31
> 
>     Number   Major   Minor   RaidDevice State
>        0       8        1        0      active sync   /dev/sda1
>        1       0        0        1      removed
> 
> 
> I should then be able to re-add sdb1, no?
> 
> # mdadm /dev/md2 --re-add /dev/sdb1
> 
> mdadm: --re-add for /dev/sdb1 to /dev/md2 is not possible
> 
> Since man mdadm explicitly provides following as example:
> 
> "mdadm /dev/md0 -f /dev/hda1 -r /dev/hda1 -a /dev/hda1"
> 
> Let's try just adding it instead of re-adding
> 
> # mdadm /dev/md2 -a /dev/sdb1
> mdadm: /dev/sdb1 reports being an active member for /dev/md2, but a --re-add fails.
> mdadm: not performing --add as that would convert /dev/sdb1 in to a spare.
> mdadm: To make this a spare, use "mdadm --zero-superblock /dev/sdb1" first.
> 
> I am perplexed as to why might this be?  I must be missing something
> pretty basic here, else I can provide additional detail as require.
> 
> Thanks for your help-- Ken
> 

btw - I  do get it that /dev/sdb1 still thinks it's an active member
of /dev/md2:

# mdadm -E /dev/sdb1
/dev/sdb1:
          Magic : a92b4efc
        Version : 1.0
    Feature Map : 0x0
     Array UUID : e3a5c30e:3fb61039:397992ff:6cc70600
           Name : archiso:2
  Creation Time : Thu Apr 19 15:56:18 2012
     Raid Level : raid1
   Raid Devices : 2

 Avail Dev Size : 524264 (256.03 MiB 268.42 MB)
     Array Size : 524264 (256.03 MiB 268.42 MB)
   Super Offset : 524272 sectors
          State : clean
    Device UUID : 68d2dc69:03f902eb:9c8ca454:27bd3854

    Update Time : Fri Apr 20 09:46:57 2012
       Checksum : 2fa4f67c - correct
         Events : 17


   Device Role : Active device 1
   Array State : AA ('A' == active, '.' == missing)


And that I could --zero-superblock and then add the partition back into
the array.

And also that from searching the web that others encountering this seem
to solve the issue by utilizing an write bitmap. 

But from my reading of the documentation I should not have to, no?

Thanks in advance for helping me understand what's actually going on
here.


-- 
Ken Gunderson <kgunders@xxxxxxxxxxxx>

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