(help!) MD RAID6 won't --re-add devices?

[Bart Kus <me@bartk.us>]

Hello,

I had a Port Multiplier failure overnight.  This put 5 out of 10 drives 
offline, degrading my RAID6 array.  The file system is still mounted 
(and failing to write):

Buffer I/O error on device md4, logical block 3907023608
Filesystem "md4": xfs_log_force: error 5 returned.
etc...

The array is in the following state:

/dev/md4:
         Version : 1.02
   Creation Time : Sun Aug 10 23:41:49 2008
      Raid Level : raid6
      Array Size : 15628094464 (14904.11 GiB 16003.17 GB)
   Used Dev Size : 1953511808 (1863.01 GiB 2000.40 GB)
    Raid Devices : 10
   Total Devices : 11
     Persistence : Superblock is persistent

     Update Time : Wed Jan 12 05:32:14 2011
           State : clean, degraded
  Active Devices : 5
Working Devices : 5
  Failed Devices : 6
   Spare Devices : 0

      Chunk Size : 64K

            Name : 4
            UUID : da14eb85:00658f24:80f7a070:b9026515
          Events : 4300692

     Number   Major   Minor   RaidDevice State
       15       8        1        0      active sync   /dev/sda1
        1       0        0        1      removed
       12       8       33        2      active sync   /dev/sdc1
       16       8       49        3      active sync   /dev/sdd1
        4       0        0        4      removed
       20       8      193        5      active sync   /dev/sdm1
        6       0        0        6      removed
        7       0        0        7      removed
        8       0        0        8      removed
       13       8       17        9      active sync   /dev/sdb1

       10       8       97        -      faulty spare
       11       8      129        -      faulty spare
       14       8      113        -      faulty spare
       17       8       81        -      faulty spare
       18       8       65        -      faulty spare
       19       8      145        -      faulty spare

I have replaced the faulty PM and the drives have registered back with 
the system, under new names:

sd 3:0:0:0: [sdn] Attached SCSI disk
sd 3:1:0:0: [sdo] Attached SCSI disk
sd 3:2:0:0: [sdp] Attached SCSI disk
sd 3:4:0:0: [sdr] Attached SCSI disk
sd 3:3:0:0: [sdq] Attached SCSI disk

But I can't seem to --re-add them into the array now!

# mdadm /dev/md4 --re-add /dev/sdn1 --re-add /dev/sdo1 --re-add 
/dev/sdp1 --re-add /dev/sdr1 --re-add /dev/sdq1
mdadm: add new device failed for /dev/sdn1 as 21: Device or resource busy

I haven't unmounted the file system and/or stopped the /dev/md4 device, 
since I think that would drop any buffers either layer might be 
holding.  I'd of course prefer to lose as little data as possible.  How 
can I get this array going again?

PS: I think the reason "Failed Devices" shows 6 and not 5 is because I 
had a single HD failure a couple weeks back.  I replaced the drive and 
the array re-built A-OK.  I guess it still counted the failure since the 
array wasn't stopped during the repair.

Thanks for any guidance,

--Bart

PPS: mdadm - v3.0 - 2nd June 2009
PPS: Linux jo.bartk.us 2.6.35-gentoo-r9 #1 SMP Sat Oct 2 21:22:14 PDT 
2010 x86_64 Intel(R) Core(TM)2 Quad CPU @ 2.40GHz GenuineIntel GNU/Linux
PPS:  # mdadm --examine /dev/sdn1
/dev/sdn1:
           Magic : a92b4efc
         Version : 1.2
     Feature Map : 0x0
      Array UUID : da14eb85:00658f24:80f7a070:b9026515
            Name : 4
   Creation Time : Sun Aug 10 23:41:49 2008
      Raid Level : raid6
    Raid Devices : 10

  Avail Dev Size : 3907023730 (1863.01 GiB 2000.40 GB)
      Array Size : 31256188928 (14904.11 GiB 16003.17 GB)
   Used Dev Size : 3907023616 (1863.01 GiB 2000.40 GB)
     Data Offset : 272 sectors
    Super Offset : 8 sectors
           State : clean
     Device UUID : c0cf419f:4c33dc64:84bc1c1a:7e9778ba

     Update Time : Wed Jan 12 05:39:55 2011
        Checksum : bdb14e66 - correct
          Events : 4300672

      Chunk Size : 64K

    Device Role : spare
    Array State : A.AA.A...A ('A' == active, '.' == missing)