RAID is preferred to bring redundancy and it saves the data if any disk fails. Currently I have CentOS 7 server which have two hard disk attached to it namely /dev/sda and /dev/sdb. Both drives are of identical in size. You can easily see your RAID 1 configuration by checking details in /proc/mdstat

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Now say if my one drive /dev/sdb fails then you can see error message on the screen or you can see error message in /var/log/messages file.

You can also use command to check the RAID status which will show that RAID is degraded and disk is removed from the RAID

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As our secondary disk is failed I will remove that disk and will add new secondary disk to the server. System named that drive as /dev/sdb we can see our new disk in fdisk -l

As our first disks have two partition and two RAID devices we will create same partitions on our secondary disk. Our primary disk have two partitions /dev/sda1 and /dev/sda2 so we will create same partitions on the secondary disk /dev/sdb

Creating /dev/sdb1

Creating sdb2

Run command partprobe so kernel can read new partition scheme

You will now need to add the appropriate partitions in RAID configuration. Please make sure that newly created partitions have same size of your primary disks partitions

Once you added this partitions in there raid rebuild process will start automatically

Just wait for RAID rebuild process and once RAID is rebuilt you are back on your track.

In the previous articles of this RAID series you went from zero to RAID hero. We reviewed several software RAID configurations and explained the essentials of each one, along with the reasons why you would lean towards one or the other depending on your specific scenario.

In this guide we will discuss how to rebuild a software RAID array without data loss when in the event of a disk failure. For brevity, we will only consider a RAID 1 setup – but the concepts and commands apply to all cases alike.

RAID Testing Scenario

Before proceeding further, please make sure you have set up a RAID 1 array following the instructions provided in Part 3 of this series: How to set up RAID 1 (Mirror) in Linux.

The only variations in our present case will be:

1) a different version of CentOS (v7) than the one used in that article (v6.5), and
2) different disk sizes for /dev/sdb and /dev/sdc (8 GB each).

In addition, if SELinux is enabled in enforcing mode, you will need to add the corresponding labels to the directory where you’ll mount the RAID device. Otherwise, you’ll run into this warning message while attempting to mount it:

SELinux RAID Mount Error

You can fix this by running:

Setting up RAID Monitoring

There is a variety of reasons why a storage device can fail (SSDs have greatly reduced the chances of this happening, though), but regardless of the cause you can be sure that issues can occur anytime and you need to be prepared to replace the failed part and to ensure the availability and integrity of your data.

A word of advice first. Even when you can inspect /proc/mdstat in order to check the status of your RAIDs, there’s a better and time-saving method that consists of running mdadm in monitor + scan mode, which will send alerts via email to a predefined recipient.

To set this up, add the following line in /etc/mdadm.conf:

In my case:

To run mdadm in monitor + scan mode, add the following crontab entry as root:

By default, mdadm will check the RAID arrays every 60 seconds and send an alert if it finds an issue. You can modify this behavior by adding the --delay option to the crontab entry above along with the amount of seconds (for example, --delay 1800 means 30 minutes).

Finally, make sure you have a Mail User Agent (MUA) installed, such as mutt or mailx. Otherwise, you will not receive any alerts.

In a minute we will see what an alert sent by mdadm looks like.

Simulating and Replacing a failed RAID Storage Device

To simulate an issue with one of the storage devices in the RAID array, we will use the --manage and --set-faulty options as follows:

This will result in /dev/sdc1 being marked as faulty, as we can see in /proc/mdstat:

Stimulate Issue with RAID Storage

More importantly, let’s see if we received an email alert with the same warning:

In this case, you will need to remove the device from the software RAID array:

Then you can physically remove it from the machine and replace it with a spare part (/dev/sdd, where a partition of type fd has been previously created):

Luckily for us, the system will automatically start rebuilding the array with the part that we just added. We can test this by marking /dev/sdb1 as faulty, removing it from the array, and making sure that the file tecmint.txt is still accessible at /mnt/raid1:

Confirm Rebuilding RAID Array

The image above clearly shows that after adding /dev/sdd1 to the array as a replacement for /dev/sdc1, the rebuilding of data was automatically performed by the system without intervention on our part.

Though not strictly required, it’s a great idea to have a spare device in handy so that the process of replacing the faulty device with a good drive can be done in a snap. To do that, let’s re-add /dev/sdb1 and /dev/sdc1:

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Recovering from a Redundancy Loss

As explained earlier, mdadm will automatically rebuild the data when one disk fails. But what happens if 2 disks in the array fail? Let’s simulate such scenario by marking /dev/sdb1 and /dev/sdd1 as faulty:

Attempts to re-create the array the same way it was created at this time (or using the --assume-clean option) may result in data loss, so it should be left as a last resort.

Let’s try to recover the data from /dev/sdb1, for example, into a similar disk partition (/dev/sde1 – note that this requires that you create a partition of type fd in /dev/sde before proceeding) using ddrescue:

Recovering Raid Array

Please note that up to this point, we haven’t touched /dev/sdb or /dev/sdd, the partitions that were part of the RAID array.

Now let’s rebuild the array using /dev/sde1 and /dev/sdf1:

Please note that in a real situation, you will typically use the same device names as with the original array, that is, /dev/sdb1 and /dev/sdc1 after the failed disks have been replaced with new ones.

In this article I have chosen to use extra devices to re-create the array with brand new disks and to avoid confusion with the original failed drives.

When asked whether to continue writing array, type Y and press Enter. The array should be started and you should be able to watch its progress with:

When the process completes, you should be able to access the content of your RAID:

Summary

In this article we have reviewed how to recover from RAID failures and redundancy losses. However, you need to remember that this technology is a storage solution and DOES NOT replace backups.

The principles explained in this guide apply to all RAID setups alike, as well as the concepts that we will cover in the next and final guide of this series (RAID management).

If you have any questions about this article, feel free to drop us a note using the comment form below. We look forward to hearing from you!