Monday, April 22, 2013

Linux RAID (RedHat,CentOS,Fedora,SELinux)

Hi Everyone,
Today, I am going to explain that how you can create a software RAID in Linux (RedHat families). In this case, I am creating a RAID-5 with 3 disks and each disk has only 1 Giga bytes capacity. As you probably know, for RAID-5, we need at least 3 disks with the same size. So, RAID-5 writes data blocks to N-1 disks, in this case 2, and parity blocks to N disk which is 3 in this case. This means that we have 2 Giga bytes to use and RAID-5 always uses one disk for parity. And you may say that we are wasting 1 Giga bytes or 1 disk here, however you protect the system against the failure of one disk. Therefore, if one disk fails, you can replace it easily by another disk without being worry about losing data. Of course, RAID-5 has its own advantages and disadvantages but it is not related to this topic now.

So, I added 3 new raw disks. I can confirm that by running the fdisk -l command (Figure 1).

                                                                                Figure 1

The md command (Multiple Disks)is used to create a software RAID. The following command builds a RAID-5 array from my 3 disks (Figure 2) and activates it:

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

                                                                               Figure 2

/proc/mdstat contains a summary of RAID/Array status. Run the following command:
cat /proc/mdstat (Figure 3)

                                                                             Figure 3

Also, take a look at the /var/log/messages file (Figure 4):

                                                                     Figure 4

Now, we need to dump the current RAID setup into a configuration file. Then, the configuration file can be read at startup or shutdown to esaily manage the array. Run the following commands (Figure 5):

echo "DEVICE /dev/sdb /dev/sdc /dev/sdd" > /etc/mdadm.conf
mdadm --detail --scan >> /etc/mdadm.conf

                                                                     Figure 5

The following command enables array to read the /etc/mdadm.conf file at startup:
mdadm -As /dev/md0

To stop the array manually, run the following command:
mdadm -S /dev/md0

To monitor our array and send notifications for any problems by email, add a MAILADDR line to the /etc/mdadm.conf. Then, start the service (Figure 6):

echo "MAILADDR root@localhost.localdomain" >> /etc/mdadm.conf
service mdmonitor start

To start mdmonitor at boot time, run the following command:
chkconfig mdmonitor on

                                                                        Figure 6

To simulate a failed disk, run the following command and then read the email notifications (Figure 7,8):
mdadm /dev/md0 -f /dev/sdc

Figure 7

Figure 8

Also, take a look at /var/log/messages file (Figure 9):

                                                                         Figure 9

To remove the disk from array and array configuration, run the following command (Figure 10):
mdadm /dev/md0 -r /dev/sdc

Now, replace the disk (if it supports Hot-swap drive hardware, otherwise turn off system) and run the following command to add the disk back to array (Figure 10):

mdadm /dev/md0 -a /dev/sdc

Take a look at the log file again (Figure 10):

                                                                         Figure 10

In order to use this RAID5 array, we need to format it and then mount it. Afterward, use df -h command to verify it (Figure 11):

                                                                             Figure 11

To mount the array at boot time, edit /etc/fstab and add the following line (Figure 12):
/dev/md0    /media/RAID5    ext4    defaults    0 0

                                                                          Figure 12

And that's all.
Hope you enjoyed.
Khosro Taraghi


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