RAID Mirroring means an exact clone (or mirror) of the same data writing to two drives. A minimum two number of disks are more required in an array to create RAID1 and it’s useful only, when read performance or reliability is more precise than the data storage capacity.
Mirrors are created to protect against data loss due to disk failure. Each disk in a mirror involves an exact copy of the data. When one disk fails, the same data can be retrieved from other functioning disk. However, the failed drive can be replaced from the running computer without any user interruption.
My Server Setup
Operating System : Ubuntu 12.04 LTS
IP Address : 192.168.0.195
Hostname : dynamicsvn
Disk 1 [1 TB] : /dev/sdb
Disk 2 [1 TB] : /dev/sdc
Step 1: Installing Prerequisites and Examine Drives
1. As I said above, we’re using mdadm utility for creating and managing RAID in Linux. So, let’s install the mdadmsoftware package on Linux using yum or apt-get package manager tool.
# yum install mdadm [on RedHat systems] # apt-get install mdadm [on Debain systems]
2. Once “mdadm” package has been installed, we need to examine our disk drives whether there is already any raid configured using the following command.
# mdadm -E /dev/sd[b-c]
mdadm : No md superblock detected on /dev/sdb
mdadm : No md superblock detected on /dev/sdc
Step 2: Drive Partitioning for RAID
3. As I mentioned above, that we’re using minimum two partitions /dev/sdb and /dev/sdc for creating RAID1. Let’s create partitions on these two drives using ‘fdisk‘ command and change the type to raid during partition creation.
# fdisk /dev/sdb
Follow the below instructions
- Press ‘n‘ for creating new partition.
- Then choose ‘P‘ for Primary partition.
- Next select the partition number as 1.
- Give the default full size by just pressing two times Enter key.
- Next press ‘p‘ to print the defined partition.
- Press ‘L‘ to list all available types.
- Type ‘t‘ to choose the partitions.
- Choose ‘fd‘ for Linux raid auto and press Enter to apply.
- Then again use ‘p‘ to print the changes what we have made.
- Use ‘w‘ to write the changes.
After ‘/dev/sdb‘ partition has been created, next follow the same instructions to create new partition on /dev/sdc drive.
# fdisk /dev/sdc
4. Once both the partitions are created successfully, verify the changes on both sdb & sdc drive using the same ‘mdadm‘ command and also confirm the RAID type as shown in the following screen grabs.
# mdadm -E /dev/sd[b-c]
Step 3: Creating RAID1 Devices
5. Next create RAID1 Device called ‘/dev/md0‘ using the following command and verity it.
# mdadm --create /dev/md0 --level=mirror --raid-devices=2 /dev/sd[b-c]1 # cat /proc/mdstat
6. Next check the raid devices type and raid array using following commands.
# mdadm -E /dev/sd[b-c]1 # mdadm --detail /dev/md0
Step 4: Creating File System on RAID Device
7. Create file system using ext4 for md0 and mount under /mnt/raid1.
# mkfs.ext4 /dev/md0
8. Next, mount the newly created filesystem under ‘/mnt/raid1‘ and create some files and verify the contents under mount point.
# mkdir /mnt/codebase # mount /dev/md0 /mnt/codebase/
9. To auto-mount RAID1 on system reboot, you need to make an entry in fstab file. Open ‘/etc/fstab‘ file and add the following line at the bottom of the file.
/dev/md0 /mnt/codebase ext4 defaults 0 0
10. Run ‘mount -a‘ to check whether there are any errors in fstab entry.
11. Next, save the raid configuration manually to ‘mdadm.conf‘ file using the below command.
# mdadm --detail --scan --verbose >> /etc/mdadm/mdadm.conf
The above configuration file is read by the system at the reboots and load the RAID devices.
Step 5: Verify Data After Disk Failure
12. Our main purpose is, even after any of hard disk fail or crash our data needs to be available. Let’s see what will happen when any of disk disk is unavailable in array.
# mdadm --detail /dev/md0
root@dynamicsvn:~# mdadm --detail /dev/md0
/dev/md0:
Version : 1.2
Creation Time : Sat Sep 26 10:53:22 2015
Raid Level : raid1
Array Size : 976760400 (931.51 GiB 1000.20 GB)
Used Dev Size : 976760400 (931.51 GiB 1000.20 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent
Update Time : Tue Sep 29 11:42:53 2015
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
Name : dynamicsvn:0 (local to host dynamicsvn)
UUID : d2087ef6:c4a7b644:40ec2320:a5aca46e
Events : 39
Number Major Minor RaidDevice State
0 8 17 0 active sync /dev/sdb1
1 8 33 1 active sync /dev/sdc1
In the above image, we can see there are 2 devices available in our RAID and Active Devices are 2. Now let us see what will happen when a disk plugged out (removed sdc disk) or fails.
# ls -l /dev | grep sd # mdadm --detail /dev/md0
Now in the above image, you can see that one of our drive is lost. I unplugged one of the drive from my Virtual machine. Now let us check our precious data.
# cd /mnt/codebase/ # ls -la dyna_repo
Did you see our data is still available. From this we come to know the advantage of RAID 1 (mirror)?