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<header><h1>Linux Software RAID</h1></header>
<main>
<p><strong>mdadm</strong> is a tool that allows for creation and
management of software RAID arrays on Linux. Creating an array
is a rather straightforward process.</p>
<h2>Install packages</h2>
<p>The only package needed is mdadm itself</p>
<pre><code>apt install mdadm</code></pre>
<h2>Partition disks</h2>
<p>We'll need to parition the disks to be used in the array before
creating it. This isn't anything complicated, we will just be
creating a single partition using all the space on each disk.</p>
<p>Use <strong>lsblk</strong> to get a list of disks attached to your system.</p>
<img src=../images/raid/lsblk.png alt="lsblk command">
<p>Then use <strong>fdisk</strong> to edit the partition table of the
first disk. In my case this would be <strong>/dev/sdb</strong>.
Be sure that you are selecting the correct disk as selecting the
wrong one can result in data being lost.</p>
<pre><code>fdisk /dev/sdb</code></pre>
<p>Use <strong>g</strong> to create a new GUID partition table.
Use <strong>n</strong> to create a new partition, and then just press
enter at all of the prompts to accept the defaults. Finally use
<strong>w</strong> to write the changes. You can use lsblk again
to verify the change and you should see that /dev/sdb now has a
partition <strong>/dev/sdb1</strong>.</p>
<img src=../images/raid/fdisk.png alt="fdisk command">
<p>Repeat this process for your other disks before continuing.</p>
<h2>Create the array</h2>
<p>Creating the array is done with a single command, but takes just a
bit of planning.</p>
<ul>
<li>Define an ID for the array, which is just a
number identifier. I use <em>0</em> in the command.</li>
<li>Determine the RAID level you want to use. I am going to
use RAID5 in this example. The argument to --level is the
number of the RAID level</li>
<li>Determine the devices that will be used in the array.
Somewhat contrary to the argument name, these devices
will actually be the partitions of the disks and not the
disks themselves. In the command, give the number of
partitions (<em>3</em>) and then a space-separated list of the
partitions that will be active in the array
(<em>/dev/sdb1 /dev/sdc1 /dev/sdd1</em>).</li>
<li>If you want to have any spare devices in the array you
will define them with the --spare-devices argument. These are
defined in the exact same way as the active RAID devices. In
the example I use <em>1</em> spare <em>/dev/sde1</em>. Spare
devices are hot-spares that will automatically be inserted into
the array if one of the disks fails.</li>
</ul>
<pre><code>mdadm --create /dev/md<em>0</em> --level=<em>5</em> --raid-devices=<em>3</em> <em>/dev/sdb1 /dev/sdc1 /dev/sdd1</em> --spare-devices=<em>1</em> <em>/dev/sde1</em></code></pre>
<p>After creating the array run the following command to get details
and the status of the array. It will take a bit to initialize the
array, you will know this is done when the state is clean. You do not
need to wait for the array to completely intialize to continue.</p>
<pre><code>mdadm --detail /dev/md<em>0</em></code></pre>
<p>Take note of the UUID and name values as we will need them in the
next step.</p>
<p>Before moving on we need to make a filesystem on the array. I'm
going to make a simple EXT4 filesystem here.</p>
<pre><code>mkfs.ext4 /dev/md<em>0</em></code></pre>
<h2>Configuration Files</h2>
<p>Open the mdadm configuration file at
<strong>/etc/mdadm/mdadm.conf</strong> and append this line. Replace
<em>uuid</em> and <em>name</em> with the values you got when running
mdadm --detail, replace <em>0</em> whatever ID you chose.</p>
<pre><code>ARRAY /dev/md<em>0</em> metadata=1.2 UUID=<em>uuid</em> name=<em>name</em></code></pre>
<p>Optionally, create an <strong>/etc/fstab</strong> entry for
automounting of the array. Replace /mnt/raid with the directory
where you want to mount the array. If you made a filesystem other than
ext4 make sure to change that value.</p>
<pre><code>/dev/md<em>0</em> /mnt/raid ext4 defaults 0 1</code></pre>
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