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Mdadm recover degraded Array
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How to Test a RAID 1 Array
Replacing a failed drive in a Linux Software RAID1 configuration (mdraid)
Rebuilding failed Linux software RAID
Software RAID Recovery on Linux
Replacing A Failed Hard Drive In A Software RAID1 Array
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I will make the partition table on sdb the same as sda. I will duplicate sda1 (/boot) as well so that if sda fails I can get it booting more quickly as well.

note: md0 doesn’t have a partition. That is really best suited to a seperate article discussing raid and lvm and so I am not going to delve in to it at this moment.

# fdisk -l
Disk /dev/sda: 320.0 GB, 320072933376 bytes
255 heads, 63 sectors/track, 38913 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          13      104391   83  Linux
/dev/sda2              14       38913   312464250   fd  Linux raid autodetect

Disk /dev/sdb: 320.0 GB, 320072933376 bytes
255 heads, 63 sectors/track, 38913 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Disk /dev/sdb doesn't contain a valid partition table

Disk /dev/md0: 319.9 GB, 319963267072 bytes
2 heads, 4 sectors/track, 78116032 cylinders
Units = cylinders of 8 * 512 = 4096 bytes

Disk /dev/md0 doesn't contain a valid partition table

# mdadm --detail /dev/md0
/dev/md0:
        Version : 00.90.03
  Creation Time : Sun Apr  8 00:22:19 2007
     Raid Level : raid1
     Array Size : 312464128 (297.99 GiB 319.96 GB)
    Device Size : 312464128 (297.99 GiB 319.96 GB)
   Raid Devices : 2
  Total Devices : 1
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Thu Aug 16 12:18:10 2007
          State : active, degraded
 Active Devices : 1
Working Devices : 1
 Failed Devices : 0
  Spare Devices : 0
UUID : 7ffa6982:50ea5134:11c17882:91cfa617
         Events : 0.960047
Number   Major   Minor   RaidDevice State
       0       8        2        0      active sync   /dev/sda2
       1       0        0        1      removed

I replace the failed hard drive with an identical hard drive. Create the partitions using the same layout if they match.

# fdisk /dev/sdb
n - new
p - primary
1 - partition number
Start 1
End 13
n - new
p - primary
2 - partition number
Start 14
End 38913
t - type
fd (Linux raid autodetect)
w - write and quit

Added the new raid partition to md0 (/dev/md0 is the mirrored raid array device)

# mdadm /dev/md0 -a /dev/sdb2
mdadm: added /dev/sdb2

# cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdb2[2] sda2[0]
      312464128 blocks [2/1] [U_]
      [>....................]  recovery =  0.3% (1072128/312464128) finish=275.0min speed=18869K/secunused devices: <none>
      
# mdadm --detail /dev/md0
/dev/md0:
        Version : 00.90.03
  Creation Time : Sun Apr  8 00:22:19 2007
     Raid Level : raid1
     Array Size : 312464128 (297.99 GiB 319.96 GB)
    Device Size : 312464128 (297.99 GiB 319.96 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Thu Aug 16 12:23:25 2007
          State : active, degraded, recovering
 Active Devices : 1
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 1
Rebuild Status : 0% complete
UUID : 7ffa6982:50ea5134:11c17882:91cfa617
         Events : 0.960629
Number   Major   Minor   RaidDevice State
       0       8        2        0      active sync   /dev/sda2
       2       8       18        1      spare rebuilding   /dev/sdb2