Creating a RAID 5 Array
The RAID 5 array type is implemented by striping data across the available devices. One component of each stripe is a calculated parity block. If a device fails, the parity block and the remaining blocks can be used to calculate the missing data. The device that receives the parity block is rotated so that each device has a balanced amount of parity information.
- Requirements: minimum of 3 storage devices
- Primary benefit: Redundancy with more usable capacity.
- Things to keep in mind: While the parity information is distributed, one disk’s worth of capacity will be used for parity. RAID 5 can suffer from very poor performance when in a degraded state.
Identify the Component Devices
To get started, find the identifiers for the raw disks that you will be using:
lsblk -o NAME,SIZE,FSTYPE,TYPE,MOUNTPOINT
The output will be something like this:
NAME SIZE FSTYPE TYPE MOUNTPOINT sda 500G disk sdb 500G disk sdc 500G disk vda 120G disk vda1 100G part / vda15 1M part
As you can see above, we have two disks without a filesystem, each 500G in size. In this example, these devices have been given the /dev/sda and /dev/sdb identifiers for this session. These will be the raw components we will use to build the array.
Create the Array
To create a RAID 5 array with these components, pass them in to the mdadm --create command. You will have to specify the device name you wish to create (/dev/md0 in our case), the RAID level, and the number of devices:
sudo mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sda /dev/sdb /dev/sdc
The mdadm tool will start to configure the array (it actually uses the recovery process to build the array for performance reasons). This can take some time to complete, but the array can be used during this time. You can monitor the progress of the mirroring by checking the /proc/mdstat file:
cat /proc/mdstat
Output
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] md0 : active raid5 sdc[3] sdb[1] sda[0] 209584128 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/2] [UU_] [===>.................] recovery = 15.6% (16362536/104792064) finish=7.3min speed=200808K/sec unused devices: <none>
As you can see in the first highlighted line, the /dev/md0 device has been created in the RAID 5 configuration using the /dev/sda, /dev/sdb and /dev/sdc devices. The second highlighted line shows the progress on the build. You can continue the guide while this process completes.
Create and Mount the Filesystem
Next, create a filesystem on the array:
sudo mkfs.ext4 -F /dev/md0
Create a mount point to attach the new filesystem:
sudo mkdir -p /mnt/md0
You can mount the filesystem by typing:
sudo mount /dev/md0 /mnt/md0
Check whether the new space is available by typing:
df -h -x devtmpfs -x tmpfs
Output
Filesystem Size Used Avail Use% Mounted on /dev/vda1 118G 2.8G 115.2G 2% / /dev/md0 597G 60M 587G 1% /mnt/md0
The new filesystem is mounted and accessible.
Save the Array Layout
To make sure that the array is reassembled automatically at boot, we will have to adjust the /etc/mdadm/mdadm.conf file.
Before you adjust the configuration, check again to make sure the array has finished assembling. Because of the way that mdadm builds RAID 5 arrays, if the array is still building, the number of spares in the array will be inaccurately reported:
cat /proc/mdstat
Output
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] md0 : active raid5 sdc[3] sdb[1] sda[0] 209584128 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU] unused devices: <none>
The output above shows that the rebuild is complete. Now, we can automatically scan the active array and append the file by typing:
sudo mdadm --detail --scan | sudo tee -a /etc/mdadm/mdadm.conf
Afterwards, you can update the initramfs, or initial RAM file system, so that the array will be available during the early boot process:
sudo update-initramfs -u
Add the new filesystem mount options to the /etc/fstab file for automatic mounting at boot:
echo '/dev/md0 /mnt/md0 ext4 defaults,nofail,discard 0 0' | sudo tee -a /etc/fstab
Your RAID 5 array should now automatically be assembled and mounted each boot.
Check how to do other raid arrays: