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Linux RAID

Here is what we are going to cover:

Use ZFS Instead: In the past I used Linux software RAID with ext4 for my personal NAS. I have since converted it to ZFS. I would recommend using ZFS if you care about your data. Take a look at these two related pages:


Ubuntu Linux RAID 10 Array Setup

The Linux software RAID subsystem allows for the creation of really excellent RAID 10 arrrays. For these instructions we are going to assume you are running Ubuntu 16.04. If you are running another distro or version package names, etc. might be different but mdadm should function mostly the same.

To start, we want to make sure we install mdadm. We run apt-get update to get a fresh view of what is on the repo. We then install the mdadm package.

apt-get update
apt-get install mdadm

This will show relevant information about block devices on the system.


This is what it looks like on my NAS after setting everything up.

Linux RAID 10 lsblk output

Create the actual array. Specify all four devices. Other numbers of drives are valid but we will use four for this example.

mdadm --create --verbose /dev/md0 --level=10 \
--raid-devices=4 /dev/sdb /dev/sdc /dev/sdd /dev/sde

As an alternative it is also possible to create a four drive array with only two disks. The other two can be added later. You would use the keyword ‘missing’ in place of the second and third disk.

mdadm --create --verbose /dev/md0 --level=10 \
--raid-devices=4 /dev/sdb missing /dev/sdc missing

Check the details of the array.

mdadm --detail /dev/md0
cat /proc/mdstat

Save everything to the config file. The path to this file might be slightly different on other distros.

mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf
cat /etc/mdadm/mdadm.conf

Create the filesystem on the array and mount it.

mkfs.ext4 -F /dev/md0
mkdir /storage1
mount /dev/md0 /storage1
df -h

Add the mount to the fstab file.

echo '/dev/md0 /storage1 ext4 defaults,nofail,discard 0 0' >> /etc/fstab

Unmount the filesystem for now.

umount /storage1
df -h

Mount it with a single argument (relying on the fstab entry).

mount /storage1
df -h

You might want to change the ownership if it will be used by another user other than root. If you had copied data onto the file system from somewhere else, you might want to do this recursively.

cd /storage1/
ls -la
chown -R user1:user1 .
ls -la

If you had left two disks out, you can add them in when ready. Add the two missing disks with the following commands.

mdadm /dev/md0 --add /dev/sdb
mdadm /dev/md0 --add /dev/sdd

Watch them get synced.

mdadm --detail /dev/md0
cat /proc/mdstat

You can use mdadm for monitoring. The command can be daemonized and can send emails, etc. This isn’t a real monitoring solution. If the daemon were to die you would silently lose your monitoring.

mdadm --monitor /dev/md0

Linux RAID - Replace a Drive

Assume we have two partitions on the drive.

Check serial number:

smartctl -a /dev/sdb|grep -i serial

Mark the drive failed

mdadm --manage /dev/md0 --fail /dev/sdb1

Check the status:

cat /proc/mdstat

Remove from the array:

mdadm --manage /dev/md0 --remove /dev/sdb1

Same for other partition:

mdadm --manage /dev/md1 --fail /dev/sdb2
mdadm --manage /dev/md1 --remove /dev/sdb2

Shut the system down:

shutdown -h now

Partition it the same as an existing disk in the array:

sfdisk -d /dev/sda | sfdisk /dev/sdb

Verify partition table:

fdisk -l

Add the new disk to the array:

mdadm --manage /dev/md0 --add /dev/sdb1
mdadm --manage /dev/md1 --add /dev/sdb2

Watch it syncing and wait until it is complete:

cat /proc/mdstat   
mdadm --detail /dev/md0

Linux - RAID 1 On The Root Disk - Setup

Install the tools you need:

apt-get install mdadm rsync initramfs-tools

Add the second disk to array:

mdadm --create /dev/md0 --level=1 --raid-devices=2 missing /dev/sdb1  
mdadm --create /dev/md1 --level=1 --raid-devices=2 missing /dev/sdb2

Format the second disk:

mkswap /dev/md1  
mkfs.ext4 /dev/md0

Add this line to the end ( not sure why ):

echo "DEVICE /dev/sda* /dev/sdb*" >> /etc/mdadm/mdadm.conf
mdadm --detail --scan             >> /etc/mdadm/mdadm.conf  

Run this:

dpkg-reconfigure mdadm

Check status:

watch -n1 cat /proc/mdstat    # watch disk sync status if syncing

Run a command to do the following:

dpkg-reconfigure grub-pc 

Copy from old drive to new one:

mkdir /tmp/mntroot  
mount /dev/md0 /tmp/mntroot  
rsync -auHxv --exclude=/proc/* --exclude=/sys/* --exclude=/tmp/* /* /tmp/mntroot/  

Edit your fstab file and swap sda or uid for raid disk:

vi /tmp/mntroot/etc/fstab     

Boot from second disk:

set root='(md/0)'  
linux /boot/vmlinuz-2.6.32-5-amd64 root=/dev/md0 ro quiet

Once the system is booted up run either of these commands to verify that your system using the RAID device as expected:

df -h

Look for something like this:

/dev/md0 on / type ext4 (rw,noatime,errors=remount-ro)

Add original disk to the array:

    mdadm /dev/md0 -a /dev/sda1  
    mdadm /dev/md1 -a /dev/sda2

Check status and watch it sync:

watch -n1 cat /proc/mdstat   

Update grub so that you will use the RAID device when you boot from either disk.

update-grub               # update grub - probably not needed
dpkg-reconfigure grub-pc  # because grub update didn't do both disks

Shutdown your system:

shutdown -h now

You should see the second drive displayed as disabled:

cat /proc/mdstat    # should show second drive as disabled

Once the system is up you can add the devices back to the array if they are missing:

mdadm /dev/md0 -a /dev/sdb1   # add it back
mdadm /dev/md1 -a /dev/sdb2   # add it back

Enable periodic RAID Checks to keep the disks in sync. These are built into the package.

dpkg-reconfigure mdadm 

You could also create a cron job that does the following:

echo "check" > /sys/block/md0/md/sync_action   

You can learn more HERE.

Disk Health with Smartmon Tools

Update repo info and install Smartmon tools.

apt-get update
apt install smartmontools

Get a list of disks. Either way works.

fdisk -l|grep -i "Disk "|grep -v ram|grep -v identifier

Check drive status.

smartctl -a /dev/sdb

Grep for errors.

smartctl -a /dev/sdb|grep -i error

Run a short test. This will run in the background and will take a few minutes. Once complete it will show in the smartctl -a’ output.

smartctl -t short /dev/sde

Wait a few minutes and check the drive again.

smartctl -a /dev/sdb

Configure smartd to run weekly SMART checks:

/dev/sda -a -d ata -o on -S on -s (S/../.././02|L/../../6/03)  
/dev/sdb -a -d ata -o on -S on -s (S/../.././02|L/../../6/03)

Linux Raid Partition Types

For MBR partition tables either of these partition types will work but I would go with FD so that the disks can be detected as RAID. There are arguments for using DA instead of FD ( see the link to kernel.org in the references section ).

For GPT you woulduse this:

Good Resources