Re: md raid recovery - perplexed

Bill Davidsen <davidsen@xxxxxxx> · Wed, 25 Apr 2012 22:50:31 -0400

Ken Gunderson wrote:

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?

I think the problem is in the documentation, it should tell you these things 

perhaps. I confess I learned about this the easy way, since I have always used a 

bitmap and not encountered this until I was testing a similar post years ago.

You might be able to --force it, but I'd rather zero the superblock myself, I 

don't like using the big hammer unless there's no "right" way. I think you do 

understand it, at least as well as I do.

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

--
Bill Davidsen <davidsen@xxxxxxx>
  "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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Re: md raid recovery - perplexed
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