When using Intel hardware fake raid there are possibilities you will be stuck at dracut shell after changing the harddisk on Centos 7. However, supposed we can still boot the server by typing ‘exit’ command. How to know if the server using fake raid? If your server using ‘Intel Rapid Storage Technology’ most probably you are using fake raid(imsm). The raid rebuilding will be done from within the OS.
To check if you are using fake raid run the below command;

[root@svr50 ~]# cat /proc/mdstat
Personalities : [raid1]
md126 : active raid1 sda[1] sdb[0]
3711641600 blocks super external:/md127/0 [2/2] [UU]

md127 : inactive sdb[1](S) sda[0](S)
10402 blocks super external:imsm

unused devices: <none>
[root@svr50 ~]#

Why it happens?
Actually, after change the harddisk of the failed raid, the UUID for the raid device will change. That is why the boot stuck at dracut shell. It just failed to boot to root device because bootloader point to the old UUID of the device. Supposed we can still boot the server by using ‘exit’ command.

Centos stuck at dracut
Centos stuck at dracut

What we need to do?
1. Change /etc/mdadm.conf to the new UUID of the raid. The below command will do, but you just need to remove the existing UUID first.

[root@svr50 ~]# mdadm --detail --scan
ARRAY /dev/md/imsm0 metadata=imsm UUID=1c8e5b8a:3df25b8c:28c3f3fa:21871fe2
ARRAY /dev/md/Volume0 container=/dev/md/imsm0 member=0 UUID=7fdecf95:90b7eab7:4ca6039a:a65fb3a3
[root@svr50 ~]#

In my case, I just comment it out. ‘cat’ is not the command to edit the file.

[root@svr29 ~]# cat /etc/mdadm.conf
# mdadm.conf written out by anaconda
AUTO +imsm +1.x -all
#ARRAY /dev/md/Volume0_0 UUID=06e7381e:1127a1dd:0f8e52f3:b8e51cc2
#ARRAY /dev/md/imsm0 UUID=ca3d7504:3f9d43a2:fe09cab6:5c4be3ed
ARRAY /dev/md/imsm0 metadata=imsm UUID=ca3d7504:3f9d43a2:fe09cab6:5c4be3ed
ARRAY /dev/md/Volume0 container=/dev/md/imsm0 member=0 UUID=c0718acf:a2049099:004a8192:7b39b033

Double check the UUID;

[root@svr50 ~]# mdadm --detail /dev/md126
         Container : /dev/md/imsm0, member 0
        Raid Level : raid1
        Array Size : 3711641600 (3.46 TiB 3.80 TB)
     Used Dev Size : 3711641600 (3.46 TiB 3.80 TB)
      Raid Devices : 2
     Total Devices : 2

             State : active
    Active Devices : 2
   Working Devices : 2
    Failed Devices : 0
     Spare Devices : 0

Consistency Policy : resync

              UUID : 7fdecf95:90b7eab7:4ca6039a:a65fb3a3
    Number   Major   Minor   RaidDevice State
       1       8        0        0      active sync   /dev/sda
       0       8       16        1      active sync   /dev/sdb

Update grub file.

[root@svr50 ~]# cat /etc/default/grub
GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
GRUB_CMDLINE_LINUX="crashkernel=auto rd.md.uuid=7fdecf95:90b7eab7:4ca6039a:a65fb3a3 rd.md.uuid=ca3d7504:3f9d43a2:fe09cab6:5c4be3ed rhgb quiet"

2. Update grub to new UUID.
Next is to update the UUID in grub bootloader.

[root@svr50 ~]# grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file …
Found linux image: /boot/vmlinuz-3.10.0-962.3.2.lve1.5.63.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-962.3.2.lve1.5.63.el7.x86_64.img
Found linux image: /boot/vmlinuz-3.10.0-962.3.2.lve1.5.44.3.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-962.3.2.lve1.5.44.3.el7.x86_64.img
Found linux image: /boot/vmlinuz-3.10.0-962.3.2.lve1.5.42.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-962.3.2.lve1.5.42.el7.x86_64.img
Found linux image: /boot/vmlinuz-3.10.0-962.3.2.lve1.5.28.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-962.3.2.lve1.5.28.el7.x86_64.img
Found linux image: /boot/vmlinuz-3.10.0-962.3.2.lve1.5.26.5.el7.x86_64
Found initrd image: /boot/initramfs-3.10.0-962.3.2.lve1.5.26.5.el7.x86_64.img
Found linux image: /boot/vmlinuz-0-rescue-40d3fa56d85044c99061b7a829c17694
Found initrd image: /boot/initramfs-0-rescue-40d3fa56d85044c99061b7a829c17694.img

3. Reboot.

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