Centos 6 software raid-1 with EFI

Assuming that you are creating new OS setup. If you already have the centos installed on disk1 and want to add disk2 as software raid-1 refer to the official RHEL/centos documents you can find how to do it.

In centos 6 setup select to manually create volumes and create the identical by size partitions on both drives. The partition type should be "Software RAID" Leave the place (200-500 MB for the /boot/efi mount point on both disks , but select as mount point only for one) after that you be able to add raid devices [create the raid volumes] from the sda and sdb previously created partitions. So it should be something like this:

   Device       mountpoint         type    
-------------------------------------------- 
HardDrivers
 /dev/sda
   /dev/sda1  /boot/efi           EFI Boot [vfat] #EFI mountpoint
   /dev/sda2  {part0}             Software RAID
   /dev/sda3                      Software RAID      
   /dev/sda4  {part1}             Software RAID
 /dev/sdb
   /dev/sdb1  {same size as sda}   EFI Boot      #no mountpoint!
   /dev/sdb2  {size -eq sda part0} Software RAID
   /dev/sdb3  {size -eq sda swap}  Software RAID
   /dev/sdb4  {size -eq sda part1} Software RAID

Note that we have specified only 1 mountpoint for EFI boot.

While you creating each volume [raid device] you be able to select partiotions that will be mirrored , filesystem type and mountpoint After you add the Raid Devices and create raid partitions the next volumes should be added at the start of the list in centos 6 install GUI. So the "picture" will be like this :

    Device       mountpoint         type    
--------------------------------------------     
RAID Devices
   /dev/md0   / {part0}          ext4
   /dev/md1                      swap             
   /dev/md2   /vz                ext4       
HardDrivers
 /dev/sda
   /dev/sda1  /boot/efi           EFI Boot [vfat] #EFI mountpoint
   /dev/sda2  {part0}             Software RAID
   /dev/sda3                      Software RAID      
   /dev/sda4  {part1}             Software RAID
 /dev/sdb
   /dev/sdb1  {same size as sda}   EFI Boot      #no mountpoint!
   /dev/sdb2  {size -eq sda part0} Software RAID
   /dev/sdb3  {size -eq sda swap}  Software RAID
   /dev/sdb4  {size -eq sda part1} Software RAID

As you can see currently EFI vfat partition not mirrored. But we have created the similar partition on sda1 and sdb1 . The mountpoint was configured only for sda1 (/boot/efi). You can press "Next" and setup will continue. Once it will done login to your server .

Check mdstat:

cat /proc/mdstat 

You should see the resync process. you can watch it live with watch command:

watch cat /proc/mdstat 

Once resync will be done you can see the output like this:

cat /proc/mdstat 
Every 2.0s: cat /proc/mdstat
Personalities : [raid1]
md2 : active raid1 sda4[0]
     1885593600 blocks super 1.1 [2/1] [U_]
     bitmap: 6/15 pages [24KB], 65536KB chunk
md0 : active raid1 sda2[0]
     51199872 blocks super 1.0 [2/1] [U_]
     bitmap: 1/1 pages [4KB], 65536KB chunk
md1 : active raid1 sdb3[1] sda3[0]
     16375808 blocks super 1.1 [2/2] [UU]
unused devices: <none>

Remember to check and resync all raid partitions (from the output below we can see that md1 is ok but md0 and md2 should be resycned (just add sdb into arr))

Now you need to make manual mirror for /boot/efi. The next copmmands copy /boot/efi from sda to sdb:

dd if=/dev/sda1 of=/dev/sdb1
efibootmgr --create --disk /dev/sdb --label "CentOS Backup" --load "\\EFI\\redhat\\grub.efi"   

The /boot/efi should be manually resynced every time you update your kernel or edit /etc/grub.conf

Now you can disable sda in BIOS/EFI and check - system should load w/o any problems.

View status of SW RAID:

 mdadm -D /dev/mdN 

Also as I can understand in case of one HDD failure - after replacement you need to initiate manual resync from survived HDD to new like this {better refer original docs}: Force Manual Resync .stop array first:

mdadm --stop /dev/mdN

Perform resync:

mdadm --assemble --run --force --update=resync /dev/mdN /dev/sdX1 /dev/sdX1

ad/remove from md: SDA1 hot remove

mdadm --manage /dev/mdN -r /dev/sda1
mdadm: hot removed /dev/sda1 from /dev/mdN

SDA add:

mdadm --manage /dev/mdN -a /dev/sda1
mdadm: added /dev/sda1

For more details about centos 6 software raid 1 install refer to the official guide : https://www.centos.org/docs/5/html/Deployment_Guide-en-US/s1-raid-config.html