How to create Software RAID 1 on Fresh NVMe Drives in CentOS/RHEL

Let’s say, you just installed two NVMe drives. That means, you currently have the following devices on your system:

/dev/nvme0n1
/dev/nvme0n2

Now, to use Raid 1 on these devices, you need to first partition them. If your devices are less than 2TB, you can use label msdos with fdisk. But I prefer gpt with parted. I will partition the disks using parted.

Open the disk nvme0n1 using parted

parted /dev/nvme0n1

Now, set the label to gpt

mklabel gpt

Now, create the primary partition

mkpart primary 0TB 1.9TB

Assuming 1.9TB is the size of your drive.

Run the above process for nvme1n1 as well. This will create one partition on each device which would be like the following:

/dev/nvme0n1p1
/dev/nvme1n1p1

Now, you may create the raid, using mdadm command as follows:

mdadm --create /dev/md201 --level=mirror --raid-devices=2 /dev/nvme0n1p1 /dev/nvme1n1p1

If you see, mdadm command not found, then you can install mdadm using the following:

yum install mdadm -y

Once done, you may now see your raid using the following command:

[[email protected] ~]# cat /proc/mdstat
Personalities : [raid1]
md301 : active raid1 sdd1[1] sdc1[0]
      976628736 blocks super 1.2 [2/2] [UU]
      bitmap: 0/8 pages [0KB], 65536KB chunk

md201 : active raid1 nvme1n1p1[1] nvme0n1p1[0]
      1875240960 blocks super 1.2 [2/2] [UU]
      bitmap: 2/14 pages [8KB], 65536KB chunk

md124 : active raid1 sda5[0] sdb5[1]
      1843209216 blocks super 1.2 [2/2] [UU]
      bitmap: 4/14 pages [16KB], 65536KB chunk

md125 : active raid1 sda2[0] sdb2[1]
      4193280 blocks super 1.2 [2/2] [UU]

md126 : active raid1 sdb3[1] sda3[0]
      1047552 blocks super 1.2 [2/2] [UU]
      bitmap: 0/1 pages [0KB], 65536KB chunk

md127 : active raid1 sda1[0] sdb1[1]
      104856576 blocks super 1.2 [2/2] [UU]
      bitmap: 1/1 pages [4KB], 65536KB chunk

unused devices: <none>

Here are a few key pieces of information about software raid:

  1. It is better not to use Raid 10 with software raid. In case the raid configuration is lost, it is hard to know which drives were set as stripe and which like a mirror by the mdadm. It is a better practice to use raid 1 as a rule of thumb with software raid.
  2. Raid 1 in mdadm doubles the read request in parallel. In raid 1, one request reads from one device, while the other request in parallel would read from the next device. This gives double read throughput when there is a parallel thread running. It still suffers from the write cost for writing data in two devices.

How to Speed Up Software RAID (mdadm) Resync Speed

mdadm is the software raid tools used in Linux system. One key problem with the software raid, is that it resync is utterly slow comparing with the existing drive speed (SSD or NVMe). The resync speed set by mdadm is default for regardless of whatever the drive type you have. To view the default values, you may run the following:

[[email protected] ~]# sysctl dev.raid.speed_limit_min
dev.raid.speed_limit_min = 1000
[[email protected] ~]# sysctl dev.raid.speed_limit_max
dev.raid.speed_limit_max = 200000

As you see, the minimum value starts from 1000 and can max upto 200K. Although, it can max upto 200K, but as min value is too low, mdadm always tries to keep the value below average to your speed available. To speed up, we would like to maximize these numbers. To change the numbers, you may run something like the following:

sysctl -w dev.raid.speed_limit_min=500000
sysctl -w dev.raid.speed_limit_max=5000000

Once done, you may now check the speed is going up immediately:

[[email protected] ~]# cat /proc/mdstat
Personalities : [raid1]
md3 : active raid1 sdb5[1] sda5[0]
      1916378112 blocks super 1.2 [2/2] [UU]
      [===============>.....]  resync = 76.2% (1461787904/1916378112) finish=27.4min speed=276182K/sec
      bitmap: 5/15 pages [20KB], 65536KB chunk

md0 : active raid1 sdb1[1] sda1[0]
      1046528 blocks super 1.2 [2/2] [UU]
      bitmap: 0/1 pages [0KB], 65536KB chunk

md2 : active raid1 sda2[0] sdb2[1]
      78576640 blocks super 1.2 [2/2] [UU]
      bitmap: 1/1 pages [4KB], 65536KB chunk

md1 : active raid1 sdb3[1] sda3[0]
      4189184 blocks super 1.2 [2/2] [UU]

unused devices: <none>

One thing to keep in mind is that, if you try to set the value too high, like we did, this might cause some handsome load on your system. If you see the load is unmanageable, you should focus on decreasing the number to something like 50k-100k for the min value.

Making The Sysctl Value Permanent

As we have established the kernel variable values on runtime, this would go back to default once we restart/reboot the server. If you want to persist the values, you need to put these values to /etc/sysctl.conf file. To make them persist, open the sysctl.conf file:

nano /etc/sysctl.conf

Add the following lines at the end of the file:

dev.raid.speed_limit_min = 500000
dev.raid.speed_limit_max = 5000000

Save the file, and run the following command:

sysctl -p

This should persist your values for those variables after the reboot as well as runtime.