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Create Software RAID under Linux system

  • Feb 19 / 2014
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Create Software RAID under Linux system

A hard drive disk is a device with limits (performance, lifetime, …) and a broken disk often leads to a data loss, and sometimes with a loss of data more or less important.
In order to avoid that kind of inconvenience, there are several hardware solutions allowing disk replication, but most of time with a too high cost for a standard use (non professional). Fortunately, there is also a software RAID, which can be set up easily and quickly without any additional hardware (assuming you already own at least 2 hard drive disks) given that it’s working on a software layer between the hardware abstraction layer and the file system.

There are different levels of RAID ; the most common levels are:

  • RAID 0 : the “striping” allows to improve performance by splitting the IO requests on several devices in parallel (2 to n disks)
  • RAID 1 : the “mirroring” allows to write same data on several disks at the same time (to disks)
  • RAID 5 : the “striping and mirroring” allows the combination of RAID 0 and RAID 1 (3 to n disks)

I will present here how to set up a RAID 1 solution (mirroring) but it will be easy to adapt the following steps if you need another RAID level.

RAID 1 setup

For the setting up, ensure you have the mdadm package installed and ready on your system. For Debian/Ubuntu, you can use the following command:

sudo apt-get install mdadm

Prepare both disks in Software Raid (type 0xfd) thanks to the fdisk tool:

sudo fdisk /dev/sdb
sudo fdisk /dev/sdc

You can now shrink and erase both disks with zeros thanks to the command below:

mdadm --zero-superblock /dev/sdb /dev/sdc

Create your RAID 1 (or any level, by changing the level attribute):

mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sdb1 /dev/sdc1

Format this new partition in ext4:

mkfs.ext4 /dev/md0

Mount your partition you just created:

mkdir /data
mount /dev/md0 /data

To get an automatic mounting point after any restart of the system, edit the file /etc/fstab  and add this following line:

/dev/md0    /data    ext4    noatime,rw    0    0

You can now check and follow your RAID status, by using following commands:

cat /proc/mdstat
mdadm --query /dev/md0

RAID Monitoring

To ensure the RAID is always working correctly and be informed when a disk is out of order, it is possible to set a monitoring solution thanks to the common tools of mdadm with a mail alert system.

Check that the monitoring settings are correctly defined in the file /etc/mdadm/mdadm.conf with the sender address (MAILFROM) and the receiver address (MAILADDR):

# instruct the monitoring daemon where to send mail alerts
MAILADDR [email protected]
MAILFROM [email protected]

You will need a mail server enabled on your server (postfix, ssmtp, …) and listening on port 25.

To check and test this configuration, ou can easily perform a test by sending a mail report thanks to this command:

sudo mdadm --monitor --scan --test --oneshot

To ensure that monitoring is enabled for your RAID devices, just check that the –monitor option is correctly set by performing the following command:

ps aux | grep mdadm
>>/sbin/mdadm --monitor --pid-file /var/run/mdadm/monitor.pid --daemonise --scan --syslog

If it’s not, you will have to add the option in the /etc/default/mdadm file (DAEMON_OPTIONS).

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