centos -- how to add a new disk into a mdadm raid5 /dev/md0 whick lost a /dev/sdc1 disk and  revoery at another machine?

fdisk -l

mdadm --stop /dev/md0

# to force active the raid by remaind 3 disks.
mdadm -A --force /dev/md0 /dev/sdb1 /dev/sde1 /dev/sdd1

# add a new disk into /dev/md0
mdadm --manage /dev/md0 --add /dev/sdc1 

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  we build a /dev/md0 on  a Unitek (removeable usb storadge with 4 disks inside) by mdadm software, and  mdadm -create /dev/md0 operated on a HUAWEI server,  our recovery work was done on my laptop ,and we meet many many problems during recovery.

we miss a disk --/dev/sdc1 from the raid /dev/md0, and now have a new disk -- same compable with the lost /dev/sbc1, on hand.

and then our joury is startiing

•  check /dev list

　　ll /dev/

　　nothing about /dev/md0 .

•  check /dev/md0 state :

　　cat /proc/mdstat

Personalities : 
unused devices: <none>

　　...nothing( for this is a  totally new machine to the /dev/md0 no conf file ,no superblock...)

•  we  try force mdadm to  assembly to force mdadm auto recovery itself:

     mdadm --assemble --run --force /dev/md0

 mdadm -A --force /dev/md0 /dev/sdb1 /dev/sde1 /dev/sdd1

      we get "can‘t open /dev/md0 array info" or "/dev/sdb1 is busying" and many other error response info;

•  then we try add a new disk into

mdadm --manage /dev/md0 --add /dev/sdc1
mdadm: Cannot get array info for /dev/md0

• try get into manage mode, can‘t get config file

[root@localhost dev]# mdadm --assemble /dev/md0 --scan --force
mdadm: /dev/md0 not identified in config file.

• a key operated is coming, and lead us to a success result.
  all the steps above, we  NOT stop the /dev/md0 and get many wrong try and get many failed resulte.

mdadm --stop /dev/md0
mdadm: stopped /dev/md0

[root@localhost dev]# mdadm --assemble /dev/md0 --scan --force
mdadm: /dev/md0 not identified in config file.
[root@localhost dev]# cat /proc/mdstat 
Personalities : 
unused devices: <none>

mdadm --force start by remain 3 disks, and add a new disk into /dev/md0, and get into recovery state.

[root@localhost dev]# mdadm -A --force /dev/md0 /dev/sdb1 /dev/sde1 /dev/sdd1
mdadm: /dev/md0 has been started with 3 drives (out of 4).
[root@localhost dev]# mdadm --manage /dev/md0 --add /dev/sdc1 
mdadm: added /dev/sdc1
[root@localhost dev]# cat /proc/mdstat 
Personalities : [raid6] [raid5] [raid4] 
md0 : active raid5 sdc1[5] sdd1[0] sde1[4] sdb1[2]
      8790402048 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [U_UU]
      [>....................]  recovery =  0.0% (184776/2930134016) finish=1321.3min speed=36955K/sec
      bitmap: 0/22 pages [0KB], 65536KB chunk

unused devices: <none>

conclusion:

• remember to stop /dev/md0 at first step, then do other operates;
• mdadm create raid5 array, every raid array info were storaged at every physical disk , no matter how their position was changed, madadm can recognize the correct raid array info.
• mdadm is realy a stronger sofeware at protect our data, if use correctly  we need not worry about our data‘s safety --- if only lost ONE disk, we can find them back;

-----other relate info reference

1. http://blog.csdn.net/waityoualife/article/details/5612243

mdadm Raid5 /dev/md0 lost a disk and recovery from another machine