RAID 1 = mirrored pair of two hard disks
If one disk fail, the raid will continue to be operational.
Read performance will be increased for parallel read operations.
Write performance will be likely the same (almost the same).

• Two hard disks of same size.
• mdadm should be installed.

yum install mdadm

mkfs.xfs -f -L DISK1 /dev/sda

-f = force
-L = hdd label

mkfs.xfs -f -L DISK2 /dev/sdb

mdadm --verbose --create /dev/md0 --level=raid1 --raid-devices=2 /dev/sda /dev/sdb

mkfs.xfs /dev/md0

mkdir /data1

mount /dev/md0 /data1

Edit /etc/fstab and add:

/dev/md0 /data1 xfs defaults 0 0

mdadm --detail --scan --verbose >> /etc/mdadm.conf

mdadm -D /dev/md0

/dev/md0:
           Version : 1.2
     Creation Time : Mon Apr 13 03:07:50 2020
        Raid Level : raid1
        Array Size : 3906886464 (3725.90 GiB 4000.65 GB)
     Used Dev Size : 3906886464 (3725.90 GiB 4000.65 GB)
      Raid Devices : 2
     Total Devices : 2
       Persistence : Superblock is persistent

     Intent Bitmap : Internal

       Update Time : Mon Apr 13 15:24:47 2020
             State : clean, resyncing
    Active Devices : 2
   Working Devices : 2
    Failed Devices : 0
     Spare Devices : 0

Consistency Policy : bitmap

     Resync Status : 90% complete

              Name : medjavaro:0  (local to host medjavaro)
              UUID : 257d9837:5eda9e27:59a51c12:d139b615
            Events : 15822

    Number   Major   Minor   RaidDevice State
       0       8       16        0      active sync   /dev/sda
       1       8       32        1      active sync   /dev/sdb

Suppose the /dev/sdb hard disk failed.
To replace it with a good hdd, set it as failed and remove it from the array:

mdadm /dev/md0 --fail /dev/sdb --remove /dev/sdb

Add the new device to the array:

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

After reinserting the hard disk, it may be mounted now on a different device letter.
Ckeck it by running:

fdisk -l

Output:

...
Disk /dev/sdd: 4000.8 GB, 4000787029504 bytes, 7814037167 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

After identifying it, re-add it to the raid array with:

mdadm --manage /dev/md0 --add /dev/sdd

mdadm --grow /dev/md0 --size=max
xfs_growfs /data1