mdadm

Setting up a RAID Array

Before you begin, ensure you have the necessary drives identified. You can use lsblk to list your block devices:

lsblk

This will output a list of your disks, including their size and partition information. Let’s assume we want to create a RAID1 (mirroring) array using /dev/sdb and /dev/sdc. Both drives should be the same size and ideally, have no partitions or data.

Creating the RAID1 Array:

First, we create the MD array:

sudo mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb /dev/sdc

This command does the following:

• sudo mdadm: Invokes the mdadm utility with superuser privileges.
• --create /dev/md0: Creates a new MD array named /dev/md0. You can choose a different name.
• --level=1: Specifies RAID level 1 (mirroring).
• --raid-devices=2: Indicates that two drives will participate in the array.
• /dev/sdb /dev/sdc: Specifies the drives to be included in the array.

Checking the Array Status:

After creating the array, verify its status:

sudo mdadm --detail /dev/md0

This command provides detailed information about the array, including its status, devices, and configuration. You should see a “state : clean” message indicating successful creation. It will also take some time to synchronize the drives. You can monitor the progress with:

cat /proc/mdstat

Formatting and Mounting the RAID Array:

Once the synchronization is complete, format the array with your desired filesystem (e.g., ext4):

sudo mkfs.ext4 /dev/md0

Finally, create a mount point and mount the array:

sudo mkdir /mnt/raid1
sudo mount /dev/md0 /mnt/raid1

Managing the RAID Array

Adding a Drive to a RAID Array (RAID1):

mdadm allows for adding drives to existing arrays, enhancing redundancy. Let’s say you want to add /dev/sdd to your existing RAID1 array /dev/md0. Ensure /dev/sdd is the same size as the others.

sudo mdadm --add /dev/md0 /dev/sdd

This command adds /dev/sdd to the /dev/md0 array. The array will then resynchronize to include the new drive.

Removing a Drive from a RAID Array (RAID1):

Removing a drive from a RAID1 array should be done with caution. While you can remove one of the drives, you lose redundancy. It can be useful for replacing a failing disk.

sudo mdadm --remove /dev/md0 /dev/sdb  # Replace /dev/sdb with the drive you want to remove

After removing a drive from a RAID1 array, the remaining drive will continue to function, but the array is now vulnerable. You must replace the failed drive as soon as possible.

Other RAID Levels:

mdadm supports various RAID levels. For example, to create a RAID5 array (data striping with parity) using three drives (/dev/sdb, /dev/sdc, /dev/sdd):

sudo mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd

Remember to adjust the number of --raid-devices according to the number of drives and the chosen RAID level. Always consult the mdadm man page (man mdadm) for detailed information and options. Consider the performance and redundancy needs of your data before choosing a RAID level.

Monitoring Your RAID Array

Regular monitoring of your RAID array is important to ensure data integrity. Use the cat /proc/mdstat command periodically to check the array’s status. Any errors or warnings should be addressed promptly. You can also use monitoring tools to automatically track your array’s health.