Identify and replace a failed disk in Linux RAID

EDIT 05/05/2020 – I realised this guide is a little confusing so hopefully this is easier to follow.

How Do I Tell If A Hard Disk Has Failed?

If a disk has failed, you will probably find a lot of error messages in the log files, e.g. /var/log/messages or /var/log/syslog and if you set up email alerts you will also receive email alerts.

You can also run cat /proc/mdstat and instead of the string [UU] you will see [U_] if you have a degraded RAID1 array.

The steps are: fail the disk, remove it, install the new disk, partition it to match the old one and then add it using the mdadm utility.

Step 1:
For this example assume /dev/sdb has failed, and /dev/sdb has two partitions /dev/sdb1 and /dev/sdb2.

Using the ‘mdadm’ utility, fail the first partition (/dev/sdb1) which is part of /dev/md0:

mdadm --manage /dev/md0 --fail /dev/sdb1

Step 2:

Using the ‘mdadm’ utility, fail the second partition (/dev/sdb2) which is part of /dev/md1:

mdadm --manage /dev/md1 --fail /dev/sdb2

Step 3:

Using the ‘mdadm’ utility, remove the disk from the array.

mdadm --manage /dev/md0 --remove /dev/sdb1

Output: mdadm: hot removed /dev/sdb1

mdadm --manage /dev/md1 --remove /dev/sdb2

Output: mdadm: hot removed /dev/sdb2

Step 4:

Replace the physical disk, and partition it exactly the same as the disk you removed – or – just copy the layout of the remaining disk.

Set the new partitions to ‘Linux raid autodetect’

Step 5:

Using the ‘mdadm’ utility, add the new disk / partitions to the array.

mdadm --manage /dev/md0 --add /dev/sdb1

Output: mdadm: re-added /dev/sdb1

mdadm --manage /dev/md1 --add /dev/sdb2

Output: mdadm: re-added /dev/sdb2

Once you have added the partitions back to the array, the rebuild process will automatically begin.

You can monitor this process with the command cat /proc/mdstat which will show the rebuild progress.

Only one partition will rebuild at a time, so you will have to wait for both to complete before the job is finished.

If the RAID is bootable, you will need to reinstall grub – ensure it is installed to *both* disks.

On a Debian system, the best way to do this is to use ‘dpkg-reconfigure’ like this dpkg-reconfigure grub-pc

One final note – don’t forget to ensure the new partition you created has the bootable flag set.

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Old method below.
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1 Preliminary Note

In this example I have two hard drives, /dev/sda and /dev/sdb, with the partitions /dev/sda1 and /dev/sda2 as well as /dev/sdb1 and /dev/sdb2.

/dev/sda1 and /dev/sdb1 make up the RAID1 array /dev/md0.
/dev/sda2 and /dev/sdb2 make up the RAID1 array /dev/md1.
/dev/sda1 + /dev/sdb1 = /dev/md0
/dev/sda2 + /dev/sdb2 = /dev/md1
/dev/sdb has failed, and we want to replace it.

2 How Do I Tell If A Hard Disk Has Failed?

If a disk has failed, you will probably find a lot of error messages in the log files, e.g. /var/log/messages or /var/log/syslog.

You can also run

cat /proc/mdstat

and instead of the string [UU] you will see [U_] if you have a degraded RAID1 array.

3 Removing The Failed Disk

To remove /dev/sdb, we will mark /dev/sdb1 and /dev/sdb2 as failed and remove them from their respective RAID arrays (/dev/md0 and /dev/md1).

First we mark /dev/sdb1 as failed:

mdadm --manage /dev/md0 --fail /dev/sdb1

The output of

cat /proc/mdstat

should look like this:

server1:~# cat /proc/mdstat
 Personalities : [linear] [multipath] [raid0] [raid1] [raid5] [raid4] [raid6] [raid10]
 md0 : active raid1 sda1[0] sdb1[2](F)
       24418688 blocks [2/1] [U_]

 md1 : active raid1 sda2[0] sdb2[1]
       24418688 blocks [2/2] [UU]

 unused devices: 

Then we remove /dev/sdb1 from /dev/md0:

mdadm --manage /dev/md0 --remove /dev/sdb1

The output should be like this:

server1:~# mdadm --manage /dev/md0 --remove /dev/sdb1
 mdadm: hot removed /dev/sdb1

And

cat /proc/mdstat

Should show this:

server1:~# cat /proc/mdstat
 Personalities : [linear] [multipath] [raid0] [raid1] [raid5] [raid4] [raid6] [raid10]
 md0 : active raid1 sda1[0]
       24418688 blocks [2/1] [U_]

 md1 : active raid1 sda2[0] sdb2[1]
       24418688 blocks [2/2] [UU]

 unused devices: 

Now we do the same steps again for /dev/sdb2 (which is part of /dev/md1):

mdadm –manage /dev/md1 –fail /dev/sdb2

Examine the RAID status with ‘cat /proc/mdstat’

server1:~# cat /proc/mdstat
 Personalities : [linear] [multipath] [raid0] [raid1] [raid5] [raid4] [raid6] [raid10]
 md0 : active raid1 sda1[0]
       24418688 blocks [2/1] [U_]

 md1 : active raid1 sda2[0] sdb2[2](F)
       24418688 blocks [2/1] [U_]

 unused devices: 

mdadm --manage /dev/md1 --remove /dev/sdb2

server1:~# mdadm --manage /dev/md1 --remove /dev/sdb2
 mdadm: hot removed /dev/sdb2

Once again examine the RAID status with ‘cat /proc/mdstat’

server1:~# cat /proc/mdstat
 Personalities : [linear] [multipath] [raid0] [raid1] [raid5] [raid4] [raid6] [raid10]
 md0 : active raid1 sda1[0]
       24418688 blocks [2/1] [U_]

 md1 : active raid1 sda2[0]
       24418688 blocks [2/1] [U_]

 unused devices: 

Now power down the system:

shutdown -h now

and replace the old /dev/sdb hard drive with a new one (it must have at least the same size as the old one – if it’s only a few MB smaller than the old one then rebuilding the arrays will fail).

4 Adding The New Hard Disk

After you have changed the hard disk /dev/sdb, boot the system.

The first thing we must do now is to create the exact same partitioning as on /dev/sda. We can do this with one simple command:

sfdisk -d /dev/sda | sfdisk /dev/sdb

You can run

fdisk -l

to check if both hard drives have the same partitioning now.

Next we add /dev/sdb1 to /dev/md0 and /dev/sdb2 to /dev/md1:

mdadm --manage /dev/md0 --add /dev/sdb1

server1:~# mdadm --manage /dev/md0 --add /dev/sdb1
 mdadm: re-added /dev/sdb1

mdadm --manage /dev/md1 --add /dev/sdb2 

server1:~# mdadm --manage /dev/md1 --add /dev/sdb2
 mdadm: re-added /dev/sdb2

Now both arrays (/dev/md0 and /dev/md1) will be synchronized. Run

cat /proc/mdstat
to see when it’s finished.

During the synchronization the output will look like this:

server1:~# cat /proc/mdstat
 Personalities : [linear] [multipath] [raid0] [raid1] [raid5] [raid4] [raid6] [raid10]
 md0 : active raid1 sda1[0] sdb1[1]
       24418688 blocks [2/1] [U_]
       [=>...................]  recovery =  9.9% (2423168/24418688) finish=2.8min speed=127535K/sec

 md1 : active raid1 sda2[0] sdb2[1]
       24418688 blocks [2/1] [U_]
       [=>...................]  recovery =  6.4% (1572096/24418688) finish=1.9min speed=196512K/sec

 unused devices: 

When the synchronization is finished, the output will look like this:

server1:~# cat /proc/mdstat
 Personalities : [linear] [multipath] [raid0] [raid1] [raid5] [raid4] [raid6] [raid10]
 md0 : active raid1 sda1[0] sdb1[1]
       24418688 blocks [2/2] [UU]

 md1 : active raid1 sda2[0] sdb2[1]
       24418688 blocks [2/2] [UU]

 unused devices: 

That’s it, you have successfully replaced /dev/sdb!

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