I recently found my home file server in a sorry state. My backup hard drive had died as well as one of the two disks in the main RAID 1 (mirrored) array where all my files and music are being served from. It was time to fix things up and ensure I’d be alerted when things failed in the future.

Below are some notes on the steps I took to create a new RAID 1 array (I had some funky partitioning on the old one and didn’t want to just replace the failed disk), safely copy my data to the new array, and setup smartmontools to monitor the disks and alert me when a disk started to fail.


The first thing I did was install the smartmontools package to test the existing drives. I knew /dev/sdc was having problems so I used the following commands to get more information.

First, install the package: sudo apt-get install smartmontools.

smartctl -i /dev/sdc gives high level overview of drive. The output should show SMART support is: Enabled in order for smartmontools to monitor the drive. Otherwise SMART can be enabled via sudo smartctl -s on /dev/sdc.

smartctl -c /dev/sdc gives test time estimates. smartctl -t long /dev/sdc runs long test. A short test can also be run. smartctl -l selftest /dev/sdc will show the test result stats.

More detail can be seen about the disk with smartctl -a /dev/sdc. This will show recent errors, test results, and other stats.

I found that /dev/sdc in my server was failing with read errors during tests. The other disk in the array was just as old as the failing disk so I decided to replace that as well as a precaution.

I also wanted to configure smartmontools as a daemon to run tests and email root when there are issues. To enable that:

  • Uncomment start_smartd=yes from /etc/default/smartmontools
  • Ensure line exists in /etc/smartd.conf
    DEVICESCAN -d removable -n standby -m root -M exec /usr/share/smartmontools/smartd-runner
  • smartd will now monitor the disks after a reboot, but it can be started immediately as well with sudo service smartmontools start.

Recovery Plan

  1. Capture a fresh backup
  2. Get a checksum of everything on the partition
    find --type f --exec md5sum "{}" + > data-old.chk

    This took quite a while to run over 30k files.

  3. Use smartctl -i /dev/sdc to get the SN of the bad disk
  4. Shutdown and pull the bad disk. Replace with a fresh disk and a second fresh disk in the empty bay
  5. Create new partitions on the 2 new disks and create a new RAID 1 array that uses the “whole” disk. I actually lefte 100MB at the end of the disk after reading about some issues with manufacturer size variability when replacing disks.
  6. mount old data on /mnt/data-old and new array on /mnt/data-new. rsync -avP /mnt/data-old/* /mnt/data-new/
  7. cd /mnt/data-new; md5sum -c data-old.chk > data-old-check.out & to verify all data is there and matches the old partition. This wasn’t really necessary when using rsync, but I was paranoid.
  8. Remount /mnt/data-new in it’s normal spot.
  9. Destroy old RAID devices and remove remaining old disk

Creating the new RAID device with mdadm

Initialize the disk by running sudo parted -a optimal /dev/sdc. In the parted prompt I created a new partition table with mklabel gpt, then I made a new partition taking up the entire disk but leaving 100MB at the end in case of future replacement disks not being the exact same size, mkpart primary 1MiB -100MiB. This procedure was then repeated on the other new disk and they were ready to be used for a new RAID 1 array.

The new array was created with:

sudo mdadm --create --verbose /dev/md0 --level=mirror --raid-devices=2 /dev/sdc1 /dev/sdd1

mdadm will then have to copy over all the garbage from one disk to the other, even though the disks are “empty”, to ensure they are mirrors of each other. This is monitored with sudo cat /proc/mdstat and the watch command if you’re into that type of thing. This took many many hours with my 2TB disks.

It’s also a good idea to put the output of sudo mdadm --detail --scan in /etc/mdadm/mdadm.conf.

Once I had all the data transferred and verified on the new array it was time to remove the old RAID device:

# Be sure to unmount the device first
sudo mdadm --stop /dev/md127
sudo mdadm --remove /dev/md127

The old drive was then removed and physically destroyed.