REPAIRING WITH MDADM

09/01/2021 - mdadm,linux,kb,storage

When adding a new – or replacing an old – drive, sometimes you need to verify what the dev is inside of Linux.

# lsblk

Will give you a breakdown of how each block device is used (or, not). In the example to the right, you can see that /dev/sdg is empty. Before we go about adding it, let’s double-check our array

# mdadm --detail /dev/md127

Gives a detailed view of the specified array

# mdadm --add /dev/md127 /dev/sdg

Add to the specified array the specified drive. This will add /dev/sdg to the array /dev/md127. You should see a response that the drive was added:

mdadm: added /dev/sdg

After this point, the array should start rebuilding by itself; you can verify with another:

# mdadm --detail /dev/mds127

The State should include recovering, and you should see your listed device something akin to:

spare rebuilding /dev/sdg

If, for whatever reason it doesn’t, you can manually grow the array.

# mdadm --grow --raid-devices=10 /dev/md127

As long as the array isn’t in a recovery/resync state, this should work. However, this can take a long time to do (days+) – as such, it is advisable to do a backup incase of, say, a power failure:

# mdadm --grow --raid-devices=10 --backup-file=/root/md127_date_grow.bak /dev/md127

Displays mdadm status on a refresh interval – helpful to keep a gaze on the recovery process of the array.

# watch cat /proc/mdstat