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

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)