Raid1 and mdadm

I have a home server full of old disks that I use for random storage purposes, the most important of which are 2 disks that make up a raid1 10TB storage area for backups.

The raid1 is built using mdadm. Its old tech and I should perhaps move to ZFS or BTRFS but currently it is what it is.

I recently heard a disk making an odd clunking noise.. it was not loud but it was persistent. I removed the drives 1 by 1 to find the culpret and sadly it was one of the 2 expensive 10TB disks.

Once removed I still had all my data but it was only on 1 disk. Before that failed I needed to replace its mirror so my data was 2 disks again.

To do this I needed to:

• get replacement disk of 10TB (or higher)
• copy partition table from existing disk
• add the disk to the raid1 array
• wait for the raid to rebuild (copy data back onto the mirror)

Here are some commands I found very useful

Get info on the raid array:

mdadm --detail /dev/md127

Dump partition table from existing raid1 disk (sda) to the replacement (sdc)

sfdisk -d /dev/sda | sfdisk /dev/sdc

Add the replacement disk to the raid1 array

mdadm --manage /dev/md127 --add /dev/sdc1

Check the progress of the rebuild

cat /proc/mdstat