How to replace a failed harddisk in Linux software RAID

This guide shows how to remove a failed hard drive from a Linux RAID1 array (software RAID), and how to add a new hard disk to the RAID1 array without losing data. I will use gdisk to copy the partition scheme, so it will work with large harddisks with GPT (GUID Partition Table) too.

 

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: <none>

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: <none>

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

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

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: <none>

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

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

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: <none>

Then 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 the command sgdisk from the gdisk package. If you havent installed gdisk yet, run this command to install it on Debian and Ubuntu:

apt-get install gdisk

For RedHat based Linux distributions like CentOS use:

yum install gdisk

and for OpenSuSE use:

yast install gdisk

The next step is optional but recomended. To ensure that you have a backup of the partition scheme, you can use sgdisk to write the partition schemes of both disks into a file. I will store the backup in the /root folder.

sgdisk --backup=/root/sda.partitiontable /dev/sda
sgdisk --backup=/root/sdb.partitiontable /dev/sdb

In case of a failure you can restore the partition tables with the --load-backup option of the sgdisk command.

Now copy the partition scheme from /dev/sda to /dev/sdb run:

sgdisk -R /dev/sdb /dev/sda

afterwards, you have to randomize the GUID on the new hard disk to ensure that they are unique

sgdisk -G /dev/sdb

You can run

sgdisk -p /dev/sda
sgdisk -p /dev/sdb

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 arays (/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: <none>

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: <none>

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

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Comments

From: Alexander

Thanks for the tutorial. There's a typo somewhere ('sdgisk' or something). But otherwise, great ;)

(BTW: It's pretty sad that such a complicated process is necessary to replace a faulty disk. That's why Linux will never catch up as a desktop operating system ;))

From: Lance

Awesome! Worked perfectly for me.  Thanks!

From: Mike

That optional step to save a copy of the backup table saved my behind after making a syntax error.  Thank you so much for the tutorial.  For what its worth, I was able to restore the partition table by running this series of commands (which I'm sure can be shortened): 

sgdisk /dev/sdc

Press R for recovery

Press L to restore partition table from backup

Type backup location (your's may differ) /root/sdc.partitiontable

Press W to write partition table to disk and exit