Replacing A Failed Hard Drive In A Software RAID1 Array

Credit goes to Author:falko

Replacing A Failed Hard Drive In A Software RAID1 Array. 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.

NOTE: There is a new version of this tutorial available that uses gdisk instead of sfdisk to support GPT partitions.

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

[sourcecode language="bash"]
cat /proc/mdstat
[/sourcecode]

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:

[sourcecode language="bash"]
mdadm --manage /dev/md0 --fail /dev/sdb1
[/sourcecode]

The output of

cat /proc/mdstat

should look like this:

[sourcecode language="bash"]
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:
[/sourcecode]

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

[sourcecode language="bash"]
mdadm --manage /dev/md0 --remove /dev/sdb1
[/sourcecode]
The output should be like this:

[sourcecode language="bash"]
server1:~# mdadm --manage /dev/md0 --remove /dev/sdb1
mdadm: hot removed /dev/sdb1
[/sourcecode]
And

cat /proc/mdstat
should show this:

[sourcecode language="bash"]
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:
[/sourcecode]

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

[sourcecode language="bash"]
mdadm --manage /dev/md1 --fail /dev/sdb2
[/sourcecode]
then,

cat /proc/mdstat

[sourcecode language="bash"]
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:
[/sourcecode]

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

[sourcecode language="bash"]
server1:~# mdadm --manage /dev/md1 --remove /dev/sdb2
mdadm: hot removed /dev/sdb2
[/sourcecode]

Now,
cat /proc/mdstat

[sourcecode language="bash"]
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:
[/sourcecode]

Then power down the system:

[sourcecode language="bash"]
shutdown -h now
[/sourcecode]

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:

[sourcecode language="bash"]
fdisk -d /dev/sda | sfdisk /dev/sdb
[/sourcecode]
You can run

[sourcecode language="bash"]
fdisk -l
[/sourcecode]
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

[sourcecode language="bash"]
server1:~# mdadm --manage /dev/md0 --add /dev/sdb1
mdadm: re-added /dev/sdb1
[/sourcecode]

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

[sourcecode language="bash"]
server1:~# mdadm --manage /dev/md1 --add /dev/sdb2
mdadm: re-added /dev/sdb2
[/sourcecode]
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:

[sourcecode language="bash"]
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:
[/sourcecode]

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

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

unused devices:
[/sourcecode]
That's it, you have successfully replaced /dev/sdb!