How to replace the failed array disk on Software Raid

#Follow the below steps to configure and add the new disk to array.

#We can use this command to create the partitions from the other disk(sda) to (sdb):

(testvm:root)$sfdisk -d /dev/sda | sfdisk /dev/sdb
Checking that no-one is using this disk right now ...
OK

Disk /dev/sdb: 8924 cylinders, 255 heads, 63 sectors/track

sfdisk: ERROR: sector 0 does not have an msdos signature
 /dev/sdb: unrecognized partition table type
Old situation:
No partitions found
New situation:
Units = sectors of 512 bytes, counting from 0

   Device Boot    Start       End   #sectors  Id  System
/dev/sdb1   *        63    208844     208782  fd  Linux raid autodetect
/dev/sdb2        208845  62990864   62782020  fd  Linux raid autodetect
/dev/sdb3             0         -          0   0  Empty
/dev/sdb4             0         -          0   0  Empty
Successfully wrote the new partition table

Re-reading the partition table ...

If you created or changed a DOS partition, /dev/foo7, say, then use dd(1)
to zero the first 512 bytes:  dd if=/dev/zero of=/dev/foo7 bs=512 count=1
(See fdisk(8).)
(testvm:root)$

#We have to run the below command to check if both hard drives have the same partitions.

(testvm:root)$ /sbin/fdisk -l

Disk /dev/sda: 36.4 GB, 36401479680 bytes
255 heads, 63 sectors/track, 4425 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *           1          13      104391   fd  Linux raid autodetect
/dev/sda2              14        3921    31391010   fd  Linux raid autodetect

Disk /dev/sdb: 73.4 GB, 73407488000 bytes
255 heads, 63 sectors/track, 8924 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *           1          13      104391   fd  Linux raid autodetect
/dev/sdb2              14        3921    31391010   fd  Linux raid autodetect

Disk /dev/md1: 32.1 GB, 32144293888 bytes
2 heads, 4 sectors/track, 7847728 cylinders
Units = cylinders of 8 * 512 = 4096 bytes

Disk /dev/md1 doesn't contain a valid partition table

Disk /dev/md0: 106 MB, 106823680 bytes
2 heads, 4 sectors/track, 26080 cylinders
Units = cylinders of 8 * 512 = 4096 bytes

Disk /dev/md0 doesn't contain a valid partition table
(testvm:root)$

#Next we should add /dev/sdb1 to /dev/md0 and /dev/sdb2 to /dev/md1 using below commands.

(testvm:root)$ mdadm --manage /dev/md0 --add /dev/sdb1
mdadm: added /dev/sdb1
(testvm:root)$ mdadm --manage /dev/md1 --add /dev/sdb2
mdadm: added /dev/sdb2
(testvm:root)$

#Now both arrays (/dev/md0 and /dev/md1) will start synchronizing.

#It will take longer time depending on amount of data to synchronize.

#output should be as below when its finished.

(testvm:root)$ cat /proc/mdstat
Personalities : [raid1]
md0 : active raid1 sdb1[1] sda1[0]
      104320 blocks [2/2] [UU]

md1 : active raid1 sdb2[1] sda2[0]
      31390912 blocks [2/2] [UU]

unused devices: <none>
(testvm:root)$


#Start the grub shell to add the grub bootblock to the replaced disk

#You can use the grub geometry command to view disk information:
(testvm:root)$ grub
Probing devices to guess BIOS drives. This may take a long time.


    GNU GRUB  version 0.97  (640K lower / 3072K upper memory)

 [ Minimal BASH-like line editing is supported.  For the first word, TAB
   lists possible command completions.  Anywhere else TAB lists the possible
   completions of a device/filename.]
grub> geometry (hd0)
geometry (hd0)
drive 0x80: C/H/S = 4425/255/63, The number of sectors = 71096640, /dev/sda
   Partition num: 0,  Filesystem type is ext2fs, partition type 0xfd
   Partition num: 1,  Filesystem type unknown, partition type 0xfd
grub> geometry (hd1)
geometry (hd1)
drive 0x81: C/H/S = 8924/255/63, The number of sectors = 143374000, /dev/sdb
   Partition num: 0,  Filesystem type is ext2fs, partition type 0xfd
   Partition num: 1,  Filesystem type unknown, partition type 0xfd

#Add the bootblock to /dev/sdb:

grub> device (hd1) /dev/sdb
device (hd1) /dev/sdb
grub> root (hd1,0)
root (hd1,0)
 Filesystem type is ext2fs, partition type 0xfd
grub> setup (hd1)
setup (hd1)
 Checking if "/boot/grub/stage1" exists... no
 Checking if "/grub/stage1" exists... yes
 Checking if "/grub/stage2" exists... yes
 Checking if "/grub/e2fs_stage1_5" exists... yes
 Running "embed /grub/e2fs_stage1_5 (hd1)"...  15 sectors are embedded.
succeeded
 Running "install /grub/stage1 (hd1) (hd1)1+15 p (hd1,0)/grub/stage2 /grub/grub.conf"... succeeded
Done.
grub> quit
quit
(testvm:root)$

#That's it, you have successfully replaced /dev/sdb!