SCENARIO:

OS: linux debian 6

Software Raid: raid 1 created with mdadm

Failed drive: sata drive (/dev/sda) of size 500Gb

New drive: same size or bigger sata drive. In my case 1Tb sata drive

WHAT TO DO:

1. Shutdown server,
2. Replace failed disk,
3. Start server,
4. Rebuild Raid 1,
5. Update grub.

LOOK FOR WORKING DISK

With fdisk I looked how is active disk in Raid 1 configured.

root@server:~# fdisk /dev/sdb 
Command (m for help): p 

Device Boot Start End Blocks Id System 
/dev/sdb1 * 1 13 104391 fd Linux raid autodetect 
/dev/sdb2 14 293 2249100 fd Linux raid autodetect 
/dev/sdb3 294 60801 486030510 fd Linux raid autodetec 

I used fdisk to create same size raid partitions on new disk:

PREPARE NEW DISK

# fdisk /dev/sda
n p 1 
n p 2
n p 3

n - new partition
p - primary partition
t - type of partition (used fd (Linux raid) for all 3 partitions on disk /dev/sda)
a - add bootable flag to first partition
w - write changes to disk

REBUILD RAID 1 with MDADM:

# mdadm /dev/md0 -a /dev/sda1 

NEW DISK:

root@server:~# fdisk /dev/sda 
Command (m for help): p 
Device Boot Start End Blocks Id System 
/dev/sda1 * 1 13 104391 fd Linux raid autodetect 
/dev/sda2 14 293 2249100 fd Linux raid autodetect 
/dev/sda3 294 60801 486030510 fd Linux raid autodetect 

BUILDING RAID 1:

root@server:~# cat /proc/mdstat 
Personalities : [raid1] 
md2 : active raid1 sda3[2] sdb3[1] 486030400 blocks [2/1] [_U] [==>..................] recovery = 14.0% (68114752/486030400) finish=147.5min speed=47214K/sec 
md1 : active raid1 sda2[0] sdb2[1] 2249024 blocks [2/2] [UU] 
md0 : active raid1 sda1[0] sdb1[1] 104320 blocks [2/2] [UU] 
unused devices: 

ALL DISKS UP AND RUNNING

root@server:~# cat /proc/mdstat Personalities : [raid1] 
md2 : active raid1 sda3[0] sdb3[1] 486030400 blocks [2/2] [UU] 
md1 : active raid1 sda2[0] sdb2[1] 2249024 blocks [2/2] [UU] 
md0 : active raid1 sda1[0] sdb1[1] 104320 blocks [2/2] [UU] 
unused devices: 

UPDATE CONFIGURATION

# mdadm --examine --scan > /etc/mdadm.conf

GRUB INSTALL/UPDATE ON NEW DISK

# grub-install /dev/sda 
# update-grub /dev/sda 

Example of howto remove a failed disk from array:

# mdadm --fail /dev/md0 /dev/sdb3
# mdadm --remove /dev/md0 /dev/sdb3 

IF PROBLEMS WITH update-grub … update-grub failed with no such disk …

SOLUTION

# mv /boot/grub/device.map /boot/grub/device.map.old 
# update-grub