Long story short, for my first thread here, I have a software RAID5 array set up as follow: 4 disk devices with a linux-RAID partition on each. Those disks are: /dev/sda1 /dev/sdb1 /dev/sdd1 /dev/sde1

/dev/md0 is the raid5 device with a ciphered LVM on it. I use cryptsetup to open the device, then vgscan and lvcan -a to map my volumes.

Yesterday, I found out that /dev/sdd1 was failing. Here are the steps I followed:

0. remove the failing disk

#  mdadm --remove /dev/md0 /dev/sdd1

1. perform a check on the faulty drive

mdadm --examine /dev/sdd1

I got the "could not read metadata" error.

2. tried to read the partition table

I used parted and discovered that my Linux-RAID partition was gone, and when I tried to re-create it (hoping to be able to re-add the drive) I got the "your device is not writable"

So, it's been clear: that hard drive is dead.

3. Extract the hard drive from my case (bad things follow)

So I tried to extract /dev/sdd1 from my case not knowing which of the 4 drives it was. So I unplugged one SATA cable to find out that I had just unplugged /dev/sde1 ; I replugged it and unplugged the following one, nice catch! it was /dev/sdd1

4. what have I done?! sad face

using :

# mdadm --detail /dev/md0

I realized that /dev/sde1 left the array marked as "removed". I tried to re-add it, not using --re-add, but :

mdadm --add /dev/md0 /dev/sde1

/proc/mdstat showed me the rebuilding process and mdadm --detail /dev/md0 displayed /dev/sde1 as "spare" ; I know I might have done something terrible here.

I tried to remove /dev/sde1 from the array and use --re-add but mdadm told me he couldn't do it and advise me to stop and reassemble the array

5. Where to go from here?

First thing first, I am waiting for a new hard drive to replace the faulty one. Once I will have it and will set it up as a new Linux-RAID partition device known as /dev/sdd1, I will have to stop the array (LVM volumes are not mounted no more, obviously, cryptsetup closed the ciphered device, yet mdadm has not been able to stop the array yet). I was thinking about rebooting the entire system and work from a clean start. Here is what I figued I should do:

# mdadm --stop /dev/md0
# mdadm --stop /dev/md0
# mdadm --examine /dev/sd*1
# mdadm --assemble --scan --run --verbose

I read that without --run option, mdadm wll refuse to scan the degraded array.

Best case scenario: /dev/sde1 is recognized by the re-assembling process and new /dev/sdd1 is used to repair the previous faulty one. I would not have lost any data and will be happy.

Worst, and most common, case scenario: Re-assembling the array fails to recover /dev/sde1 and I have to start from a blank new array.

Am I missing something here? What should I review from this procedure?

Best Regards from France

  • 1. stick to read only / copy-on-write overlay raid.wiki.kernel.org/index.php/… 2. re-create using said overlays unix.stackexchange.com/a/131927/30851 # if it wasn't marked spare, --stop followed by --assemble --force would have sorted it in as much that is possible after yanking the wrong drive. May 14, 2018 at 15:44
  • greetings @frostschutz ; sadly, after the --add action on /dev/sde1, the drive was marked as spare by mdadm --detail /dev/md0 so I am guessing from your reply that I screwed my data May 14, 2018 at 16:33

1 Answer 1


So, I managed to get a full recovery, thanks to this link

What I did is as follow:

  1. I replaced the faulty disk and restarted the server.
  2. Then, I formatted the new disk as a Linux-RAID partition type.

    # mdadm --examine /dev/sda1 /dev/sdb1 /dev/sdd1 /dev/sde1

Then, based on the link above, I (re)created the array, based on the infos given by the --examine command.

# mdadm --create /dev/md0 --level=5 --raid-devices=4 --chunk=512 --name=server:0 /dev/sda1 /dev/sdb1 missing /dev/sde1 --assume-clean

As stated on this link, the --assume-clean did the trick! It avoided the "spare" state from /dev/sde1 and used it as a active part of the new array.

Key thing upon re-creating the array from "existing" devices might be not to mess up with the chunk parameter, unless you will loose the data.

  1. I then added the new device to this new array:

    # mdadm --add /dev/md0 /dev/sde1

The server started rebuilding (took 6hrs for 10 Tb), and after this, I forced an integrity check on the whole array (which took 6 hrs aswell)

I recovered everything and I am quite relieved!

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.