[CentOS] Accidentally nuked my system - any suggestions ?

Wed Dec 5 07:31:05 UTC 2018
Phil Perry <pperry at elrepo.org>

On 05/12/2018 05:37, Nicolas Kovacs wrote:
> Le 04/12/2018 à 23:50, Stephen John Smoogen a écrit :
>> In the rescue mode, recreate the partition table which was on the sdb
>> by copying over what is on sda
>> sfdisk –d /dev/sda | sfdisk /dev/sdb
>> This will give the kernel enough to know it has things to do on
>> rebuilding parts.
> Once I made sure I retrieved all my data, I followed your suggestion,
> and it looks like I'm making big progress. The system booted again,
> though it feels a bit sluggish. Here's the current state of things.
> [root at alphamule:~] # cat /proc/mdstat
> Personalities : [raid1]
> md125 : active raid1 sdb2[1] sda2[0]
>        512960 blocks super 1.0 [2/2] [UU]
>        bitmap: 0/1 pages [0KB], 65536KB chunk
> md126 : inactive sda1[0](S)
>        16777216 blocks super 1.2
> md127 : active raid1 sda3[0]
>        959323136 blocks super 1.2 [2/1] [U_]
>        bitmap: 8/8 pages [32KB], 65536KB chunk
> unused devices: <none>
> Now how can I make my RAID array whole again? For the record, /dev/sda
> is intact, and /dev/sdb is the faulty disk. How can I force
> synchronization with /dev/sda?
> Cheers,
> Niki

If you are confident in the state of sda, I would remove sdb from the 
array, copy the partition table from sda to sdb as Stephen suggested 
earlier, then add sdb back to the array and allow the data to be synced:

For example:

mdadm --fail /dev/md125 /dev/sdb2
mdadm --remove /dev/md125 /dev/sdb2

mdadm --fail /dev/md126 /dev/sdb1
mdadm --remove /dev/md126 /dev/sdb1

mdadm --fail /dev/md127 /dev/sdb3
mdadm --remove /dev/md127 /dev/sdb3

sfdisk –d /dev/sda | sfdisk /dev/sdb

then add them back and watch then rebuild:

mdadm --add /dev/md125 /dev/sdb2
mdadm --add /dev/md126 /dev/sdb1
mdadm --add /dev/md127 /dev/sdb3

After they have all resynced, I would flush the device buffers for good 
measure. For example:

blockdev --flushbufs /dev/sdb1

Lastly, don't forget to reinstall grub to sdb:

grub2-install --recheck /dev/sdb